Semiconductor device

ABSTRACT

A semiconductor device is provided, which includes a switch having a first transistor and a logic circuit having an output terminal. The logic circuit includes a bootstrap circuit having at least one second transistor. The bootstrap circuit is electrically connected to the output terminal. The first transistor and the second transistor have the same conductivity type. Each of the first transistor and the second transistor includes an oxide semiconductor layer including a channel formation region and a pair of gate electrodes with the oxide semiconductor layer provided therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including atransistor and a method for driving the semiconductor device.

2. Description of the Related Art

A process of fabricating a semiconductor device such as a shift registercan be simplified when the semiconductor device is composed oftransistors of the same conductivity type, for example, composed of onlyn-channel transistors, as compared to when the semiconductor device iscomposed of p-channel transistors and n-channel transistors. A circuitthat is composed of transistors of the same conductivity type and has afunction similar to that of a complementary MOS (CMOS) is sometimesreferred to as “unipolar CMOS”. A shift register composed of transistorsof the same conductivity type is disclosed in Patent Document 1, forexample.

FIG. 7 is a circuit diagram illustrating the configuration of part ofthe shift register disclosed in Patent Document 1. In the shiftregister, a plurality of stages 80 illustrated in FIG. 7 can becascaded. The stage 80 includes a transistor 81, a transistor 82, atransistor 83, a transistor 84, and a capacitor 85. All the transistors(the transistors 81 to 84) included in the stage 80 can be n-channeltransistors. Thus, all the transistors included in the shift registercan be n-channel transistors.

In the stage 80, a drain of the transistor 81 is connected to a terminalC1 and supplied with a clock signal CLK. A source of the transistor 81is connected to an output terminal OUT and a drain of the transistor 82.A signal output from the output terminal OUT is an output signal of thestage 80. A gate of the transistor 81 is connected to a source of thetransistor 83. A source of the transistor 82 is connected to a terminalVSS and supplied with a low power supply potential (e.g., a groundpotential). A gate of the transistor 82 is connected to a terminal C2and supplied with an inverted clock signal CLKB. The inverted clocksignal CLKB is an inversion signal of the clock signal CLK (a signalhaving a logic level opposite to that of the clock signal CLK). A gateand a drain of the transistor 83 are connected to an input terminal N.The input terminal N is supplied with an output signal of the previousstage 80. The source of the transistor 83 is connected to a drain of thetransistor 84. A source of the transistor 84 is connected to theterminal VSS and supplied with the low power supply potential (e.g., theground potential). A gate of the transistor 84 is connected to theoutput terminal OUT of the subsequent stage 80. The capacitor 85 isprovided between the gate and source of the transistor 81.

In the stage 80, a high-level potential of the output signal is ahigh-level potential of the clock signal CLK, and a low-level potentialof the output signal is the low power supply potential. The shiftregister including the plurality of stages 80 drives a plurality ofloads using output signals that are sequentially output from theplurality of stages 80. For example, when the shift register is used ina scan line driver circuit of a display device, the loads correspond toscan lines, elements connected to the scan lines, and the like.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2006-024350

SUMMARY OF THE INVENTION

In the shift register disclosed in Patent Document 1, the high-levelpotential of the output signal is the high-level potential of the clocksignal CLK. That is, a load (corresponding to an element driven by theshift register, a wiring to which a signal is input from the shiftregister, or the like) is driven with the high-level potential of theclock signal CLK. Consequently, a circuit that generates the clocksignal CLK (hereinafter also referred to as “clock signal generator”)requires high current drive capability. In order to enhance the currentdrive capability of the circuit, it is necessary to increase the size ofelements constituting the circuit (e.g., the channel width and length oftransistors) and to provide a buffer constituted of a large-sizedelement, for example, which results in the increase in area of thecircuit. In view of the above, an object of one embodiment of thepresent invention is to provide a shift register that can operate with aclock signal generator with low current drive capability.

Note that the description of the object does not disturb the existenceof other objects. An object other than the above will be apparent fromand can be derived from the description in the specification, thedrawings, the claims, and the like.

A semiconductor device according to one embodiment of the presentinvention includes a switch and a logic circuit having an input terminalto which an input signal is input through the switch in the on state.Note that the logic circuit is a circuit (e.g., an inverter circuit)that inverts the logic level of a signal input to the input terminal (orone of a plurality of input terminals) and outputs the inverted signalfrom the output terminal. The switch is turned on or off by a clocksignal (or an inversion signal of the clock signal). The logic circuitis supplied with a high power supply potential from a wiring(hereinafter also referred to as “high power supply line”), and suppliedwith a low power supply potential from another wiring (hereinafter alsoreferred to as “low power supply line”). The logic circuit inverts thelogic level of a signal input to the input terminal (or one of aplurality of input terminals) and outputs the inverted signal from theoutput terminal by electrically connecting the output terminal to thehigh power supply line and/or the low power supply line. The output fromthe logic circuit is to be the output from the semiconductor device.Accordingly, a load (e.g., a bus line or an element connected to a busline) driven by the semiconductor device is driven while beingelectrically connected to the high power supply line or the low powersupply line. A clock signal generator may be included in thesemiconductor device or may be an external circuit provided outside thesemiconductor device. Note that the semiconductor device and a load maybe collectively considered as a semiconductor device.

Here, all transistors included in the semiconductor device can have thesame conductivity type. That is, transistors forming the switch and thelogic circuit can all have the same conductivity type. In that case, thelogic circuit includes a bootstrap circuit and corrects the outputsignal by using the bootstrap circuit. In other words, n-channeltransistors can be used as all the transistors included in thesemiconductor device, whereby the logic circuit can output the low powersupply potential from the output terminal when the input signal is ahigh-level potential and output the high power supply potential from theoutput terminal by raising the potential of the output terminal with thebootstrap circuit when the input signal is a low-level potential.Alternatively, p-channel transistors can be used as all the transistorsincluded in the semiconductor device, whereby the logic circuit canoutput the high power supply potential from the output terminal when theinput signal is a low-level potential and output the low power supplypotential from the output terminal by lowering the potential of theoutput terminal with the bootstrap circuit when the input signal is alow-level potential.

Further, the logic circuit can have a plurality of input terminals (aninput terminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The logic circuit can be configured to invertthe logic level of the input signal input to the input terminal andoutput the inverted signal from the output terminal by controllingelectrical connection between one of the high power supply line and thelow power supply line and the output terminal with a signal input to theinverted input terminal, and by controlling electrical connectionbetween the other of the high power supply line and the low power supplyline and the output terminal with a signal input to the input terminal.For example, when the transistors included in the logic circuit aren-channel transistors, the logic circuit can be configured to invert thelogic level of the input signal input to the input terminal and outputthe inverted signal from the output terminal in the following manner: agate of a transistor provided between the high power supply line and theoutput terminal is electrically connected to the inverted input terminalsuch that the on/off state of the transistor is controlled by a signalinput to the inverted input terminal, and a gate of a transistorprovided between the low power supply line and the output terminal iselectrically connected to the input terminal such that the on/off stateof the transistor is controlled by a signal input to the input terminal.Alternatively, for example, when the transistors included in the logiccircuit are p-channel transistors, the logic circuit can be configuredto invert the logic level of the input signal input to the inputterminal and output the inverted signal from the output terminal in thefollowing manner: a gate of a transistor provided between the low powersupply line and the output terminal is electrically connected to theinverted input terminal such that the on/off state of the transistor iscontrolled by a signal input to the inverted input terminal, and a gateof a transistor provided between the high power supply line and theoutput terminal is electrically connected to the input terminal suchthat the on/off state of the transistor is controlled by a signal inputto the input terminal.

The transistor included in the semiconductor device includes asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith a first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with a second gateinsulating layer placed therebetween. Here, one of the pair of gateelectrodes is referred to as a gate of the transistor and the otherthereof is referred to as a backgate of the transistor. The other of thepair of gate electrodes (backgate) can be electrically connected to asource of the transistor. Alternatively, the other of the pair of gateelectrodes (backgate) can be electrically connected to the low powersupply line when the transistor is an n-channel transistor, whereas theother of the pair of gate electrodes (backgate) can be electricallyconnected to the high power supply line when the transistor is ap-channel transistor. Note that one of the pair of gate electrodes(gate) may by electrically connected to the other of the pair of gateelectrodes (backgate) such that they are collectively used as a gate ofthe transistor.

A plurality of transistors included in the semiconductor device eachinclude a semiconductor layer in which a channel is formed and a pair ofgate electrodes between which the semiconductor layer is sandwiched. Oneof the pair of gate electrodes overlaps with the semiconductor layerwith a first gate insulating layer placed therebetween, and the otherthereof overlaps with the semiconductor layer with a second gateinsulating layer placed therebetween. It is possible that one of thepair of gate electrodes serves as the gate and the other thereof servesas the backgate in one or more of the transistors included in thesemiconductor device, and that one of the pair of gate electrodes servesas the backgate and the other thereof serves as the gate in the othertransistors included in the semiconductor device. That is, thepositional relation between the gate and backgate of one or more of thetransistors can be opposite to that of the other transistors included inthe semiconductor device. For example, some of the transistors includedin the semiconductor device can be bottom-gate transistors in which thegate is provided below the semiconductor layer, and the othertransistors included in the semiconductor device can be top-gatetransistors in which the gate is provided above the semiconductor layer.The backgate of the transistor can be electrically connected to thesource thereof. Alternatively, the backgate can be electricallyconnected to the low power supply line when the transistor is ann-channel transistor, and electrically connected to the high powersupply line when the transistor is a p-channel transistor. Note that thegate and the backgate may be electrically connected to each other suchthat they are collectively used as a gate of the transistor.

For example, a semiconductor device according to one embodiment of thepresent invention includes a switch and a logic circuit to which aninput signal is input through the switch in the on state. Transistorsforming the switch and the logic circuit are n-channel transistors. Theswitch is turned on or off by a clock signal. The logic circuit includesa bootstrap circuit, an input terminal to which the input signal isinput, an inverted input terminal to which a signal with a logic levelopposite to that of the input signal is input, and an output terminal.The logic circuit is supplied with a high power supply potential from ahigh power supply line, and supplied with a low power supply potentialfrom a low power supply line. Electrical connection between the highpower supply line and the output terminal is controlled by a signalinput to the inverted input terminal and electrical connection betweenthe low power supply line and the output terminal is controlled by asignal input to the input terminal, whereby the low power supplypotential is output from the output terminal when the input signal is ahigh-level potential, whereas when the input signal is a low-levelpotential, the high power supply potential is output from the outputterminal by raising a potential of the output terminal with thebootstrap circuit. Each of the n-channel transistors includes asemiconductor layer in which a channel is formed, and a pair of gateelectrodes provided so that the semiconductor layer is placedtherebetween. One of the pair of gate electrodes of the n-channeltransistor overlaps with the semiconductor layer with a first gateinsulating layer placed therebetween, and the other of the pair of gateelectrodes overlaps with the semiconductor layer with a second gateinsulating layer placed therebetween and is electrically connected to asource of the n-channel transistor.

For example, a semiconductor device according to another embodiment ofthe present invention includes a switch and a logic circuit to which aninput signal is input through the switch in the on state. Transistorsforming the switch and the logic circuit are p-channel transistors. Theswitch is turned on or off by a clock signal. The logic circuit includesa bootstrap circuit, an input terminal to which the input signal isinput, an inverted input terminal to which a signal with a logic levelopposite to that of the input signal is input, and an output terminal.The logic circuit is supplied with a high power supply potential from ahigh power supply line, and supplied with a low power supply potentialfrom a low power supply line. Electrical connection between the lowpower supply line and the output terminal is controlled by a signalinput to the inverted input terminal and electrical connection betweenthe high power supply line and the output terminal is controlled by asignal input to the input terminal, whereby the high power supplypotential is output from the output terminal when the input signal is alow-level potential, whereas when the input signal is a high-levelpotential, the low power supply potential is output from the outputterminal by lowering a potential of the output terminal with thebootstrap circuit. Each of the p-channel transistors includes asemiconductor layer in which a channel is formed, and a pair of gateelectrodes provided so that the semiconductor layer is placedtherebetween. One of the pair of gate electrodes of the p-channeltransistor overlaps with the semiconductor layer with a first gateinsulating layer placed therebetween, and the other of the pair of gateelectrodes overlaps with the semiconductor layer with a second gateinsulating layer placed therebetween and is electrically connected to asource of the p-channel transistor.

In the transistor, the semiconductor layer in which the channel isformed can be formed using an oxide semiconductor. Alternatively, thesemiconductor layer may be formed using silicon, such as amorphoussilicon, polycrystalline silicon, or single crystal silicon.

The semiconductor device according to one embodiment of the presentinvention may be a display device, such as a display device including aliquid crystal element or a display device including a light-emittingelement such as an electroluminescent (EL) element.

The semiconductor device according to one embodiment of the presentinvention may be an image sensor.

The semiconductor device according to one embodiment of the presentinvention may be an arithmetic circuit or a memory device. Note thatCPUs and programmable LSIs are included in the category of arithmeticcircuits.

One embodiment of the present invention can be an electronic deviceincluding any of the above semiconductor devices.

The semiconductor device according to one embodiment of the presentinvention includes the switch and the logic circuit having the inputterminal to which an input signal is input through the switch in the onstate. The switch is turned on or off by a clock signal (or an inversionsignal of the clock signal). The logic circuit inverts the logic levelof the input signal and outputs the inverted signal from the outputterminal by electrically connecting the output terminal to the highpower supply line and/or the low power supply line. The output from thelogic circuit is to be the output from the semiconductor device.Accordingly, a load driven by the semiconductor device is driven whilebeing electrically connected to the high power supply line or the lowpower supply line. The semiconductor device having such a structure doesnot drive a load by using a high-level potential (or a low-levelpotential) of the clock signal, and thus a clock signal generator doesnot need high current drive capability. Consequently, the area of theclock signal generator can be reduced.

When all the transistors included in the semiconductor device have thesame conductivity type, the process of fabricating the semiconductordevice can be simplified. Thus, the yield can be increased and the costscan be reduced. In that case, the logic circuit is configured to correctthe output signal by using a bootstrap circuit. In such a manner, thelogic circuit constituted of transistors of the same conductivity typecan output an output signal with an amplitude voltage that is(substantially) equal to the power supply voltage (corresponding to adifference between the high power supply potential and the low powersupply potential).

Further, the logic circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The logic circuit can be configured to invertthe logic level of an input signal input to the input terminal andoutput the inverted signal from the output terminal by controllingelectrical connection between the output terminal and one of the highpower supply line and the low power supply line with a signal input tothe inverted input terminal, and by controlling electrical connectionbetween the output terminal and the other of the high power supply lineand the low power supply line with a signal input to the input terminal.Accordingly, in the logic circuit composed of transistors of the sameconductivity type, one of the transistor provided between the high powersupply line and the output terminal and the transistor provided betweenthe low power supply line and the output terminal can be turned on,while the other of these two transistors can be turned off. As a result,the through current in a circuit provided between the high power supplyline and the low power supply line can be suppressed.

The transistor included in the semiconductor device includes asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith a first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with a second gateinsulating layer placed therebetween. The other of the pair of gateelectrodes (backgate) of the transistor can be electrically connected toa source thereof. Alternatively, the other of the pair of gateelectrodes (backgate) can be electrically connected to the low powersupply line when the transistor is an n-channel transistor, whereas theother of the pair of gate electrodes (backgate) can be electricallyconnected to the high power supply line when the transistor is ap-channel transistor. Thus, the transistor can be prevented from beingplaced in a normally-on state, resulting in reduction in malfunction ofthe semiconductor device and reduction in through current.

In such a manner, a unipolar CMOS having a function similar to that of aCMOS can be fabricated by using transistors of the same conductivitytype. In addition, the semiconductor device according to one embodimentof the present invention can drive a load (e.g., a bus line) withoutrequiring high current supply capability of a clock signal generator.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a configuration of a semiconductor device describedin Embodiment 1;

FIGS. 2A and 2B each illustrate a configuration of a logic circuit, andFIGS. 2C and 2D each illustrate a configuration of a switch;

FIG. 3 illustrates a semiconductor device including a plurality ofstages;

FIG. 4 illustrates a configuration of a semiconductor device describedin Embodiment 2;

FIG. 5 illustrates a configuration of an amplifier circuit;

FIG. 6 illustrates a configuration of a semiconductor device describedin Embodiment 3;

FIG. 7 illustrates a configuration of a conventional semiconductordevice;

FIGS. 8A and 8B illustrate an example of a structure of a transistor;

FIGS. 9A to 9E illustrate an example of a method for fabricating atransistor; and

FIGS. 10A to 10C illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be hereinafter described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that a variety of changesand modifications can be made without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be limited to the description of the embodiments below. Instructures given below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and explanation thereof will not be repeated.

Note that the size, thickness, and regions in the drawings areexaggerated for clarity in some cases. Thus, an embodiment of thepresent invention is not limited to such scales. Alternatively, drawingsare perspective views of ideal examples. Thus, an embodiment of thepresent invention is not limited to shapes illustrated in the drawingsand can include variations in shape due to a fabrication technique ordimensional deviation, for example.

Note that an explicit description “X and Y are connected” indicates thecase where X and Y are electrically connected, the case where X and Yare functionally connected, and the case where X and Y are directlyconnected. Here, each of X and Y denotes an object (e.g., a device, anelement, a circuit, a wiring, an electrode, a terminal, a conductivefilm, or a layer). Accordingly, a connection relation other than thoseshown in drawings and texts is also included without limitation to apredetermined connection relation, for example, the connection relationshown in the drawings and the texts.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a dc-dcconverter, a step-up dc-dc converter, or a step-down dc-dc converter) ora level shifter circuit for changing the potential level of a signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit that can increase signal amplitude, the amountof current, or the like, an operational amplifier, a differentialamplifier circuit, a source follower circuit, or a buffer circuit; asignal generator circuit; a memory circuit; and/or a control circuit)can be connected between X and Y. When a signal output from X istransmitted to Y, it can be said that X and Y are functionally connectedeven if another circuit is provided between X and Y.

Note that an explicit description “X and Y are connected” means that Xand Y are electrically connected, X and Y are functionally connected,and X and Y are directly connected. That is, the explicit description “Xand Y are electrically connected” is the same as an explicit simpleexpression “X and Y are connected”.

Even when independent components are electrically connected to eachother in a circuit diagram, there is the case where one conductive layerhas functions of a plurality of components (e.g., a wiring and anelectrode), such as the case where part of a wiring functions as anelectrode. The “electrical connection” in this specification includes inits category such a case where one conductive layer has functions of aplurality of components.

Embodiment 1

In Embodiment 1, a specific embodiment of a semiconductor deviceaccording to the present invention will be described with reference toFIG. 1, FIGS. 2A to 2D, and FIG. 3.

The semiconductor device can include a stage 10 illustrated in FIG. 1.The stage 10 includes an input terminal N, an inverted input terminalNB, a switch SW1, a switch SW2, a switch SW3, a switch SW4, a logiccircuit INV1, a logic circuit INV2, a logic circuit INV3, a logiccircuit INV4, an output terminal OUT, and an inverted output terminalOUTB.

The logic circuits INV1 to INV4 each have an input terminal N, aninverted input terminal NB, and an output terminal OUT and invert asignal input to the input terminal N and output the inverted signal fromthe output terminal OUT. The logic circuits INV1 to INV4 can be calledinverters. The output terminal OUT of the logic circuit NV1 is connectedto the input terminal N of the logic circuit INV3, the inverted inputterminal NB of the logic circuit INV4, and the output terminal OUT. Theoutput terminal OUT of the logic circuit INV2 is connected to theinverted input terminal NB of the logic circuit INV3, the input terminalN of the logic circuit INV4, and the inverted output terminal OUTB. Theswitches SW1 to SW4 each have a function of determining electricalcontinuity between a terminal A and a terminal B depending on a signalinput to a terminal X. Thus, the switch SW1 has a function ofdetermining electrical continuity between the input terminal N and theinput terminal N of the logic circuit INV1 and between the inputterminal N and the inverted input terminal NB of the logic circuit INV2depending on a signal input to the terminal X. The switch SW2 has afunction of determining electrical continuity between the inverted inputterminal NB and the inverted input terminal NB of the logic circuit INV1and between the inverted input terminal NB and the input terminal IN ofthe logic circuit INV2 depending on a signal input to the terminal X.The terminals X of the switches SW1 and SW2 are connected to a terminalC1 to which one of a clock signal and an inversion signal of the clocksignal (inverted clock signal) is input. The switch SW3 has a functionof determining electrical continuity between the output terminal OUT ofthe logic circuit INV3 and the input terminal N of the logic circuitINV1 and between the output terminal OUT of the logic circuit INV3 andthe inverted input terminal INB of the logic circuit INV2 depending on asignal input to the terminal X. The switch SW4 has a function ofdetermining electrical continuity between the output terminal OUT of thelogic circuit INV4 and the inverted input terminal INB of the logiccircuit INV1 and between the output terminal OUT of the logic circuitINV4 and the input terminal IN of the logic circuit INV2 depending on asignal input to the terminal X. The terminals X of the switches SW3 andSW4 are connected to a terminal C2 to which the other of the clocksignal and the inverted clock signal is input.

In the stage 10 illustrated in FIG. 1, when the switches SW1 and SW2 areturned on by the clock signal (or the inverted clock signal), theswitches SW3 and SW4 are turned off by the inverted clock signal (or theclock signal), whereas when the switches SW1 and SW2 are turned off bythe clock signal (or the inverted clock signal), the switches SW3 andSW4 are turned on by the inverted clock signal (or the clock signal).Accordingly, the stage 10 has a function of holding a signal input tothe input terminal N in synchronization with the clock signal. The stage10 illustrated in FIG. 1 can therefore be called a flip-flop circuit ora latch circuit.

One embodiment of a specific configuration of each of the logic circuitsINV1 to INV4 will be described with reference to FIGS. 2A and 2B.

The logic circuits INV1 to INV4 each can be a logic circuit NVillustrated in FIG. 2A. The logic circuit NV illustrated in FIG. 2Aincludes a transistor 101, a transistor 102, a transistor 103, and acapacitor 211. A gate of the transistor 101 is connected to a powersupply line V1 to which a power supply potential V1 is supplied. A drainof the transistor 101 is connected to the inverted input terminal NB. Asource of the transistor 101 is connected to a gate of the transistor102. A drain of the transistor 102 is connected to the power supply lineV1. A source of the transistor 102 is connected to the output terminalOUT. A gate of the transistor 103 is connected to the input terminal N.A source of the transistor 103 is connected to a power supply line V2 towhich a power supply potential V2 different from the power supplypotential V1 is supplied. A drain of the transistor 103 is connected tothe output terminal OUT. One of a pair of electrodes of the capacitor211 is connected to the gate of the transistor 102, and the otherthereof is connected to the source of the transistor 102. Note thatparasitic capacitance of the transistor 102 and the like can be activelyused instead of providing the capacitor 211.

The transistors 101 to 103 can be of the same conductivity type. Whenthe transistors 101 to 103 are n-channel transistors, the power supplypotential V1 is set to be higher than the power supply potential V2 andthe power supply potential V2 is, for example, a ground potential. Thatis, the power supply potential V1 is a high power supply potential andthe power supply potential V2 is a low power supply potential. On theother hand, when the transistors 101 to 103 are p-channel transistors,the power supply potential V1 is set to be lower than the power supplypotential V2 and is a ground potential, for example. That is, the powersupply potential V1 is a low power supply potential and the power supplypotential V2 is a high power supply potential.

Each of the transistors 101 to 103 includes a semiconductor layer inwhich a channel is formed and a pair of gate electrodes between whichthe semiconductor layer is sandwiched. One of the pair of gateelectrodes can overlap with the semiconductor layer with a first gateinsulating layer placed therebetween, and the other thereof can overlapwith the semiconductor layer with a second gate insulating layer placedtherebetween. Here, one of the pair of gate electrodes is called a gateof the transistor and the other thereof is called a backgate of thetransistor. The other of the pair of gate electrodes (backgate) of thetransistor can be connected to the source thereof. FIG. 2A schematicallyillustrates a configuration in which each of the transistors 101 to 103has the gate and the backgate that is connected to the source. Note thatthe other of the pair of gate electrodes (backgate) can be connected tothe power supply line V2. That is, the other of the pair of gateelectrodes (backgate) can be connected to the low power supply line whenthe transistor is an n-channel transistor, whereas the other of the pairof gate electrodes (backgate) can be connected to the high power supplyline when the transistor is a p-channel transistor. Thus, thetransistors 101 to 103 can be prevented from being placed in anormally-on state.

It is possible that one of the pair of gate electrodes serves as thegate and the other thereof serves as the backgate in one or more of thetransistors 101 to 103, and that one of the pair of gate electrodesserves as the backgate and the other thereof serves as the gate in therest of them. For example, it is possible that one of the pair of gateelectrodes serves as the gate and the other thereof serves as thebackgate in the transistor 102, and that one of the pair of gateelectrodes serves as the backgate and the other thereof serves as thegate in the transistor 103. That is, the positional relation between thegate and the backgate of the transistor 102 can be opposite to that ofthe transistor 103. For example, one of the transistors 102 and 103 canbe a bottom-gate transistor in which the gate is provided below thesemiconductor layer, and the other of the transistors 102 and 103 can bea top-gate transistor in which the gate is provided above thesemiconductor layer. The backgate of the transistor can be electricallyconnected to the source thereof. Alternatively, the backgate can beelectrically connected to the low power supply line when the transistoris an n-channel transistor, and electrically connected to the high powersupply line when the transistor is a p-channel transistor.

The operation of the logic circuit NV illustrated in FIG. 2A will bedescribed.

First, the operation of the logic circuit NV will be described assumingthat the transistors 101 to 103 are n-channel transistors. When ahigh-level potential is input to the input terminal N and a low-levelpotential is input to the inverted input terminal NB, the transistor 103is turned on and the transistor 102 is turned off. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (low power supply potential) is output from theoutput terminal OUT. When a low-level potential is input to the inputterminal N and a high-level potential is input to the inverted inputterminal NB, the transistor 103 is turned off and the transistor 102 isturned on. Thus, the output terminal OUT is connected to the powersupply line V1. Here, when the potential of the source of the transistor101 reaches a certain potential (a potential that is lower than thepower supply potential V1, which is the potential of the gate of thetransistor 101, by the threshold voltage of the transistor 101) by thehigh-level potential input to the inverted input terminal NB, thetransistor 101 is turned off and the source of the transistor 101 isbrought into a floating state. The potential of the source of thetransistor 102 keeps rising, so that the potential of the gate of thetransistor 102 is raised by capacitive coupling due to the capacitor211. Accordingly, the potential of the output terminal OUT is raised tothe power supply potential V1 (or a potential close thereto), and thepower supply potential V1 (high power supply potential) is output fromthe output terminal OUT. In other words, it can be said that the logiccircuit NV illustrated in FIG. 2A includes a bootstrap circuit.

Next, the operation of the logic circuit NV will be described assumingthat the transistors 101 to 103 are p-channel transistors. When alow-level potential is input to the input terminal N and a high-levelpotential is input to the inverted input terminal NB, the transistor 103is turned on and the transistor 102 is turned off. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (high power supply potential) is output from theoutput terminal OUT. When a high-level potential is input to the inputterminal N and a low-level potential is input to the inverted inputterminal NB, the transistor 103 is turned off and the transistor 102 isturned on. Thus, the output terminal OUT is connected to the powersupply line V1. Here, when the potential of the source of the transistor101 reaches a certain potential (a potential that is higher than thepower supply potential V1, which is the potential of the gate of thetransistor 101, by the threshold voltage of the transistor 101) by thelow-level potential input to the inverted input terminal NB, thetransistor 101 is turned off and the source of the transistor 101 isbrought into a floating state. The potential of the source of thetransistor 102 keeps decreasing, so that the potential of the gate ofthe transistor 102 is lowered by capacitive coupling due to thecapacitor 211. Accordingly, the potential of the output terminal OUT islowered to the power supply potential V1 (or a potential close thereto),and the power supply potential V1 (low power supply potential) is outputfrom the output terminal OUT. In other words, it can be said that thelogic circuit NV in FIG. 2A includes a bootstrap circuit.

As another example, the logic circuits INV1 and INV2 each can be thelogic circuit NV illustrated in FIG. 2A, and the logic circuits INV3 andINV4 each can be a logic circuit NV illustrated in FIG. 2B. The logiccircuit NV illustrated in FIG. 2B includes a transistor 104 and atransistor 105. A gate of the transistor 104 is connected to theinverted input terminal NB. A drain of the transistor 104 is connectedto the power supply line V1 to which the power supply potential V1 issupplied. A source of the transistor 104 is connected to the outputterminal OUT. A gate of the transistor 105 is connected to the inputterminal N. A source of the transistor 105 is connected to the powersupply line V2 to which the power supply potential V2 different from thepower supply potential V1 is supplied. A drain of the transistor 105 isconnected to the output terminal OUT.

The transistors 104 and 105 can be of the same conductivity type. Whenthe transistors 104 and 105 are n-channel transistors, the power supplypotential V1 is set to be higher than the power supply potential V2 andthe power supply potential V2 is, for example, a ground potential. Thatis, the power supply potential V1 is the high power supply potential andthe power supply potential V2 is the low power supply potential. On theother hand, when the transistors 104 and 105 are p-channel transistors,the power supply potential V1 is set to be lower than the power supplypotential V2 and is a ground potential, for example. That is, the powersupply potential V1 is the low power supply potential and the powersupply potential V2 is the high power supply potential.

Each of the transistors 104 and 105 includes a semiconductor layer inwhich a channel is formed and a pair of gate electrodes between whichthe semiconductor layer is sandwiched. One of the pair of gateelectrodes can overlap with the semiconductor layer with a first gateinsulating layer placed therebetween, and the other thereof can overlapwith the semiconductor layer with a second gate insulating layer placedtherebetween. Here, one of the pair of gate electrodes is called a gateof the transistor and the other thereof is called a backgate of thetransistor. The other of the pair of gate electrodes (backgate) of thetransistor can be connected to the source thereof FIG. 2B schematicallyillustrates a configuration in which each of the transistors 104 and 105has the gate and the backgate that is connected to the source. Note thatthe other of the pair of gate electrodes (backgate) can be connected tothe power supply line V2. That is, the other of the pair of gateelectrodes (backgate) can be connected to the low power supply line whenthe transistor is an n-channel transistor, whereas the other of the pairof gate electrodes (backgate) can be connected to the high power supplyline when the transistor is a p-channel transistor. Thus, thetransistors 104 and 105 can be prevented from being placed in anormally-on state.

It is possible that one of the pair of gate electrodes serves as thegate and the other thereof serves as the backgate in one of thetransistors 104 and 105, and that one of the pair of gate electrodesserves as the backgate and the other thereof serves as the gate in theother of the transistors 104 and 105. For example, it is possible thatone of the pair of gate electrodes serves as the gate and the otherthereof serves as the backgate in the transistor 104, and that one ofthe pair of gate electrodes serves as the backgate and the other thereofserves as the gate in the transistor 105. That is, the positionalrelation between the gate and the backgate of the transistor 104 can beopposite to that of the transistor 105. For example, one of thetransistors 104 and 105 can be a bottom-gate transistor in which thegate is provided below the semiconductor layer, and the other of thetransistors 104 and 105 can be a top-gate transistor in which the gateis provided above the semiconductor layer. The backgate of thetransistor can be electrically connected to the source thereof.Alternatively, the backgate can be electrically connected to the lowpower supply line when the transistor is an n-channel transistor, andelectrically connected to the high power supply line when the transistoris a p-channel transistor.

The operation of the logic circuit NV illustrated in FIG. 2B will bedescribed.

First, the operation of the logic circuit NV will be described assumingthat the transistors 104 and 105 are n-channel transistors. When ahigh-level potential is input to the input terminal N and a low-levelpotential is input to the inverted input terminal NB, the transistor 104is turned off and the transistor 105 is turned on. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (low power supply potential) is output from theoutput terminal OUT. When a low-level potential is input to the inputterminal N and a high-level potential is input to the inverted inputterminal NB, the transistor 104 is turned on and the transistor 105 isturned off. Thus, the output terminal OUT is connected to the powersupply line V1. Note that the potential output from the output terminalOUT cannot be made higher than a potential that is lower than thehigh-level potential input to the inverted input terminal NB (e.g., thepower supply potential V1) by the threshold voltage of the transistor104.

Next, the operation of the logic circuit NV will be described assumingthat the transistors 104 and 105 are p-channel transistors. When alow-level potential is input to the input terminal IN and a high-levelpotential is input to the inverted input terminal NB, the transistor 104is turned off and the transistor 105 is turned on. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (high power supply potential) is output from theoutput terminal OUT. When a high-level potential is input to the inputterminal IN and a low-level potential is input to the inverted inputterminal NB, the transistor 104 is turned on and the transistor 105 isturned off. Thus, the output terminal OUT is connected to the powersupply line V1. Note that the potential output from the output terminalOUT cannot be made lower than a potential that is higher than thelow-level potential input to the inverted input terminal NB (e.g., thepower supply potential V1) by the threshold voltage of the transistor104.

As described above, the logic circuit NV illustrated in FIG. 2B is alsoa circuit that inverts the logic level of a signal input to the inputterminal IN and outputs the inverted signal from the output terminalOUT, and thus can be called an inverter. Note that the logic circuit NVin FIG. 2B does not include a bootstrap circuit and thus cannot outputan output signal with a predetermined amplitude, which is equal to thepower supply voltage (corresponding to a difference between the highpower supply potential and the low power supply potential, that is,|V1−V2|). However, the configuration of the logic circuit NV illustratedin FIG. 2B can be simpler than that of the logic circuit NV includingthe bootstrap circuit illustrated in FIG. 2A.

Here, in FIG. 1, the outputs from the logic circuits INV3 and INV4 areinput through the switches SW3 and SW4 to the logic circuits INV1 andINV2, and the output signal from the logic circuit INV1 and the outputsignal from the logic circuit INV2 are to be the output signal from theoutput terminal OUT and the output signal from the inverted outputterminal OUTB, respectively, of each stage. For that reason, even if thelogic circuits INV3 and INV4 cannot output an output signal with anamplitude that is (substantially) equal to the power supply voltage(corresponding to a difference between the high power supply potentialand the low power supply potential, that is, |V1−V2|), the logiccircuits INV1 and INV2 amplify the outputs from the logic circuits INV3and INV4, and output signals with an amplitude that is (substantially)equal to the power supply voltage are output from the output terminalOUT and the inverted output terminal OUTB of the stage 10. Consequently,a semiconductor device that can output a signal with a predeterminedamplitude can be provided with a simpler circuit configuration.

In FIG. 1, each of the switches SW1 and SW2 can be a switch SW composedof a transistor 106 illustrated in FIG. 2C. A gate of the transistor 106can be connected to the terminal X. One of a source and a drain of thetransistor 106 can be connected to the terminal A, and the other thereofcan be connected to the terminal B. The transistor 106 includes asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith a first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with a second gateinsulating layer placed therebetween. Here, one of the pair of gateelectrodes is called a gate of the transistor and the other thereof iscalled a backgate of the transistor. The other of the pair of gateelectrodes (backgate) can be connected to the power supply line V2. FIG.2C schematically illustrates a configuration in which the transistor 106has the gate and the backgate that is connected to the power supply lineV2. That is, the other of the pair of gate electrodes (backgate) can beconnected to the low power supply line when the transistor is ann-channel transistor, whereas the other of the pair of gate electrodes(backgate) can be connected to the high power supply line when thetransistor is a p-channel transistor. Note that the other of the pair ofgate electrodes (backgate) of the transistor 106 can be connected to thesource thereof. Thus, the transistor 106 can be prevented from beingplaced in a normally-on state.

In FIG. 1, each of the switches SW3 and SW4 can be a switch SW composedof a transistor 107 illustrated in FIG. 2D. A gate of the transistor 107can be connected to the terminal X. One of a source and a drain of thetransistor 107 can be connected to the terminal A, and the other thereofcan be connected to the terminal B. The transistor 107 includes asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith a first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with a second gateinsulating layer placed therebetween. Here, one of the pair of gateelectrodes is referred to as a gate of the transistor and the otherthereof is referred to as a backgate of the transistor. FIG. 2Dschematically illustrates a configuration in which the transistor 107has the gate and the backgate that is connected to the power supply lineV2. That is, the other of the pair of gate electrodes (backgate) can beconnected to the low power supply line when the transistor is ann-channel transistor, whereas the other of the pair of gate electrodes(backgate) can be connected to the high power supply line when thetransistor is a p-channel transistor. Note that the other of the pair ofgate electrodes (backgate) of the transistor 107 can be connected to thesource thereof. Thus, the transistor 107 can be prevented from beingplaced in a normally-on state.

Here, the requisite current drive capability of the switch is lower thanthat of the logic circuit, so that the size of the transistor composedof the switch can be made smaller than that of the transistor includedin the logic circuit. That is, the channel width (W, also referred to asthe gate width) (or the ratio (W/L) of channel width to channel length(L, also referred to as the gate length)) of the transistors 106 and 107can be designed to be smaller than that of any or all of the transistors101 to 105. Thus, the increase in integration and reduction in size ofthe semiconductor device can be achieved.

Since the logic circuits INV3 and INV4 do not directly drive a loadconnected to the output terminal OUT, their requisite current drivecapability is lower than that of the logic circuits INV1 and INV2.Therefore, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors included in the logic circuitsINV3 and INV4 (the transistors 101 to 103, in particular the transistors102 and 103, or the transistors 104 and 105) can be made smaller thanthat of the transistors included in the logic circuits INV1 and INV2(the transistors 101 to 103, in particular the transistors 102 and 103).Thus, the increase in integration and reduction in size of thesemiconductor device can be achieved.

Since the transistor 101 in the logic circuit NV illustrated in FIG. 2Adoes not directly drive a load connected to the output terminal OUT, itsrequisite current drive capability is lower than that of the transistors102 and 103. For that reason, the channel width (W) (or the ratio (W/L)of channel width to channel length (L)) of the transistor 101 can bemade smaller than that of the transistors 102 and 103.

The semiconductor device according to one embodiment of the presentinvention can include a plurality of stages 10 illustrated in FIG. 1.For example, as illustrated in FIG. 3, a semiconductor device 100 caninclude a plurality of stages 10 that are cascaded so that the inputterminal N is connected to the output terminal OUT of the previous stageand the inverted input terminal NB is connected to the inverted outputterminal OUTB of the previous stage. Here, in adjacent two stages amongthe plurality of stages 10, the logic levels of signals input to theterminals C1 can be different from each other (i.e., the logic levels ofsignals input to the terminals C2 can be different from each other). Forexample, the clock signal can be input to the terminal C1 and theinverted clock signal can be input to the terminal C2 in one stage 10,and the inverted clock signal can be input to the terminal C1 and theclock signal can be input to the terminal C2 in the stage 10 next to theone stage 10. In FIG. 3, the clock signal is represented by CLK, and theinversion signal of the clock signal is represented by CLKB. Signalsinput to the input terminal IN and the inverted input terminal NB of thefirst stage among the cascaded stages 10 can be signals with oppositelogic levels. In FIG. 3, a signal SP and an inversion signal SPB of thesignal SP are input to the input terminal N and the inverted inputterminal NB, respectively, of the first stage.

The semiconductor device 100 illustrated in FIG. 3 has a function ofholding a signal that is input to the input terminal N of the firststage sequentially in the stages 10 in synchronization with the clocksignal. The semiconductor device 100 can therefore be called a shiftregister. The signal SP can be called a start pulse. The semiconductordevice 100 has a function of sequentially shifting the start pulse insynchronization with the clock signal CLK and outputting the shiftedpulses as outputs SR1, SR2, SR3 . . . . The semiconductor device 100 candrive a load with the outputs SR1, SR2, SR3 . . . . Although the outputsSR1 to SR3 are set as output signals from the output terminal OUT ofeach stage 10 here, they are not limited to these and may be outputsignals from the inverted output terminal OUTB of each stage 10, forexample. Alternatively, only outputs SR2, SR4, SR6 . . . ofeven-numbered stages 10 may be used as outputs of the semiconductordevice 100 to drive a load.

The semiconductor device shown in this embodiment includes the switches(the switches SW1 to SW4) and the logic circuits having the inputterminal to which an input signal is input through the switch in the onstate (the logic circuits INV1 to INV4). The switch is turned on or offby a clock signal (or an inversion signal of the clock signal). Thelogic circuit inverts the logic level of the input signal and outputsthe inverted signal from the output terminal by connecting the outputterminal to the high power supply line or the low power supply line. Theoutput from the logic circuit is to be the output from the semiconductordevice. Accordingly, a load driven by the semiconductor device is drivenwhile being connected to the high power supply line or the low powersupply line. The semiconductor device having such a structure does notdrive a load by using a high-level potential (or a low-level potential)of the clock signal, and thus a clock signal generator does not needhigh current drive capability. Consequently, the area of the clocksignal generator can be reduced.

When all the transistors included in the semiconductor device have thesame conductivity type, the process of fabricating the semiconductordevice can be simplified. Thus, the yield can be increased and the costscan be reduced. In that case, the logic circuits (all the logic circuitsINV1 to INV4 or only the logic circuits INV3 and INV4) are configured tocorrect the output signal by using a bootstrap circuit. In such amanner, the logic circuit constituted of transistors of the sameconductivity type can output an output signal with an amplitude voltagethat is (substantially) equal to the power supply voltage (correspondingto a difference between the high power supply potential and the lowpower supply potential).

Further, the logic circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The logic circuit can be configured to invertthe logic level of an input signal input to the input terminal andoutput the inverted signal from the output terminal by controllingconnection between the output terminal and one of the high power supplyline and the low power supply line with a signal input to the invertedinput terminal, and by controlling connection between the outputterminal and the other of the high power supply line and the low powersupply line with a signal input to the input terminal. Accordingly, inthe logic circuit composed of transistors of the same conductivity type,one of the transistor provided between the high power supply line andthe output terminal and the transistor provided between the low powersupply line and the output terminal can be turned on, while the other ofthese two transistors can be turned off. As a result, the throughcurrent in a circuit provided between the high power supply line and thelow power supply line can be suppressed.

The transistors included in the semiconductor device each include asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith the first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with the second gateinsulating layer placed therebetween. The other of the pair of gateelectrodes (backgate) of the transistor can be connected to the sourcethereof. Alternatively, the other of the pair of gate electrodes(backgate) can be connected to the low power supply line when thetransistor is an n-channel transistor, and can be connected to the highpower supply line when the transistor is a p-channel transistor. Thus,the transistors can be prevented from being placed in a normally-onstate, resulting in reduction in malfunction of the semiconductor deviceand reduction in through current.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

In Embodiment 2, a specific embodiment of a semiconductor deviceaccording to the present invention will be described with reference toFIG. 4 and FIG. 5.

The semiconductor device can include a stage 10 illustrated in FIG. 4.The stage 10 includes an input terminal N, an inverted input terminalNB, a switch SW1, a switch SW2, a switch SW3, a switch SW4, a logiccircuit INV1, a logic circuit INV2, a logic circuit INV3, a logiccircuit INV4, an amplifier circuit BUF1, an amplifier circuit BUF2, anoutput terminal OUT, and an inverted output terminal OUTB.

Each of the logic circuits INV1 to INV4 has an input terminal N, aninverted input terminal NB, and an output terminal OUT and inverts asignal input to the input terminal N and outputs the inverted signalfrom the output terminal OUT. The logic circuits INV1 to INV4 can becalled inverters. Each of the amplifier circuits BUF1 and BUF2 has aninput terminal N, an inverted input terminal NB, and an output terminalOUT and transforms (lowers) the impedance to output, from the outputterminal OUT, a signal input to the input terminal N. The outputterminal OUT of the logic circuit INV1 is connected to the inputterminal N of the amplifier circuit BUF1 and the inverted input terminalNB of the amplifier circuit BUF2. The output terminal OUT of the logiccircuit INV2 is connected to the inverted input terminal NB of theamplifier circuit BUF1 and the input terminal N of the amplifier circuitBUF2. The output terminal OUT of the amplifier circuit BUF1 is connectedto the input terminal IN of the logic circuit INV3, the inverted inputterminal NB of the logic circuit INV4, and the output terminal OUT. Theoutput terminal OUT of the amplifier circuit BUF2 is connected to theinverted input terminal NB of the logic circuit INV3, the input terminalN of the logic circuit INV4, and the inverted output terminal OUTB. Theswitches SW1 to SW4 each have a function of determining electricalcontinuity between a terminal A and a terminal B depending on a signalinput to a terminal X. Thus, the switch SW1 has a function ofdetermining electrical continuity between the input terminal N and theinput terminal N of the logic circuit INV1 and between the inputterminal N and the inverted input terminal INB of the logic circuit INV2depending on a signal input to the terminal X. The switch SW2 has afunction of determining electrical continuity between the inverted inputterminal NB and the inverted input terminal INB of the logic circuitINV1 and between the inverted input terminal NB and the input terminal Nof the logic circuit INV2 depending on a signal input to the terminal X.The terminals X of the switches SW1 and SW2 are connected to a terminalC1 to which one of a clock signal and an inversion signal of the clocksignal (inverted clock signal) is input. The switch SW3 has a functionof determining electrical continuity between the output terminal OUT ofthe logic circuit INV3 and the input terminal N of the logic circuitINV1 and between the output terminal OUT of the logic circuit INV3 andthe inverted input terminal NB of the logic circuit INV2 depending on asignal input to the terminal X. The switch SW4 has a function ofdetermining electrical continuity between the output terminal OUT of thelogic circuit INV4 and the inverted input terminal NB of the logiccircuit INV1 and between the output terminal OUT of the logic circuitINV4 and the input terminal N of the logic circuit INV2 depending on asignal input to the terminal X. The terminals X of the switches SW3 andSW4 are connected to a terminal C2 to which the other of the clocksignal and the inverted clock signal is input.

In the stage 10 illustrated in FIG. 4, when the switches SW1 and SW2 areturned on by the clock signal (or the inverted clock signal), theswitches SW3 and SW4 are turned off by the inverted clock signal (or theclock signal), whereas when the switches SW1 and SW2 are turned off bythe clock signal (or the inverted clock signal), the switches SW3 andSW4 are turned on by the inverted clock signal (or the clock signal).Accordingly, the stage 10 has a function of holding a signal input tothe input terminal N in synchronization with the clock signal. The stage10 illustrated in FIG. 4 can therefore be called a flip-flop circuit ora latch circuit.

One embodiment of a specific configuration of the amplifier circuitsBUF1 and BUF2 will be described with reference to FIG. 5.

The amplifier circuits BUF1 and BUF2 each can be an amplifier circuitBUF illustrated in FIG. 5. The amplifier circuit BUF illustrated in FIG.5 includes a transistor 108, a transistor 109, a transistor 110, and acapacitor 212. A gate of the transistor 108 is connected to a powersupply line V1 to which a power supply potential V1 is supplied. A drainof the transistor 108 is connected to the input terminal N. A source ofthe transistor 108 is connected to a gate of the transistor 109. A drainof the transistor 109 is connected to the power supply line V1. A sourceof the transistor 109 is connected to the output terminal OUT. A gate ofthe transistor 110 is connected to the inverted input terminal NB. Asource of the transistor 110 is connected to a power supply line V2 towhich a power supply potential V2 different from the power supplypotential V1 is supplied. A drain of the transistor 110 is connected tothe output terminal OUT. One of a pair of electrodes of the capacitor212 is connected to the gate of the transistor 109, and the otherthereof is connected to the source of the transistor 109. Note thatparasitic capacitance of the transistor 109 and the like can be activelyused instead of providing the capacitor 212.

The transistors 108 to 110 can be of the same conductivity type. Whenthe transistors 108 to 110 are n-channel transistors, the power supplypotential V1 is set to be higher than the power supply potential V2 andthe power supply potential V2 is, for example, a ground potential. Thatis, the power supply potential V1 is a high power supply potential andthe power supply potential V2 is a low power supply potential. On theother hand, when the transistors 108 to 110 are p-channel transistors,the power supply potential V1 is set to be lower than the power supplypotential V2 and is a ground potential, for example. That is, the powersupply potential V1 is a low power supply potential and the power supplypotential V2 is a high power supply potential.

Each of the transistors 108 to 110 includes a semiconductor layer inwhich a channel is formed and a pair of gate electrodes between whichthe semiconductor layer is sandwiched. One of the pair of gateelectrodes can overlap with the semiconductor layer with a first gateinsulating layer placed therebetween, and the other thereof can overlapwith the semiconductor layer with a second gate insulating layer placedtherebetween. Here, one of the pair of gate electrodes is called a gateof the transistor and the other thereof is called a backgate of thetransistor. The other of the pair of gate electrodes (backgate) of thetransistor can be connected to the source thereof. FIG. 5 schematicallyillustrates a configuration in which each of the transistors 108 to 110has the gate and the backgate that is connected to the source. Note thatthe other of the pair of gate electrodes (backgate) can be connected tothe power supply line V2. That is, the other of the pair of gateelectrodes (backgate) can be connected to the low power supply line whenthe transistor is an n-channel transistor, whereas the other of the pairof gate electrodes (backgate) can be connected to the high power supplyline when the transistor is a p-channel transistor. Thus, thetransistors 108 to 110 can be prevented from being placed in anormally-on state.

It is possible that one of the pair of gate electrodes serves as thegate and the other thereof serves as the backgate in one or more of thetransistors 108 to 110, and that one of the pair of gate electrodesserves as the backgate and the other thereof serves as the gate in therest of them. For example, it is possible that one of the pair of gateelectrodes serves as the gate and the other thereof serves as thebackgate in the transistor 109, and that one of the pair of gateelectrodes serves as the backgate and the other thereof serves as thegate in the transistor 110. That is, the positional relation between thegate and the backgate of the transistor 109 can be opposite to that ofthe transistor 110. For example, one of the transistors 109 and 110 canbe a bottom-gate transistor in which the gate is provided below thesemiconductor layer, and the other of the transistors 109 and 110 can bea top-gate transistor in which the gate is provided above thesemiconductor layer. The backgate of the transistor can be electricallyconnected to the source thereof. Alternatively, the backgate can beelectrically connected to the low power supply line when the transistoris an n-channel transistor, and electrically connected to the high powersupply line when the transistor is a p-channel transistor.

The operation of the amplifier circuit BUF illustrated in FIG. 5 will bedescribed.

First, the operation of the amplifier circuit BUF will be describedassuming that the transistors 108 to 110 are n-channel transistors. Whena low-level potential is input to the input terminal IN and a high-levelpotential is input to the inverted input terminal NB, the transistor 110is turned on and the transistor 109 is turned off. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (low power supply potential) is output from theoutput terminal OUT. When a high-level potential is input to the inputterminal N and a low-level potential is input to the inverted inputterminal NB, the transistor 110 is turned off and the transistor 109 isturned on. Thus, the output terminal OUT is connected to the powersupply line V1. Here, when the potential of the source of the transistor108 reaches a certain potential (a potential that is lower than thepower supply potential V1, which is the potential of the gate of thetransistor 108, by the threshold voltage of the transistor 108) by thehigh-level potential input to the input terminal N, the transistor 108is turned off and the source of the transistor 108 is brought into afloating state. The potential of the source of the transistor 109 keepsrising, so that the potential of the gate of the transistor 109 israised by capacitive coupling due to the capacitor 212. Accordingly, thepotential of the output terminal OUT is raised to the power supplypotential V1 (or a potential close thereto), and the power supplypotential V1 (high power supply potential) is output from the outputterminal OUT. In other words, it can be said that the amplifier circuitBUF illustrated in FIG. 5 includes a bootstrap circuit.

Next, the operation of the amplifier circuit BUF will be describedassuming that the transistors 108 to 110 are p-channel transistors. Whena high-level potential is input to the input terminal N and a low-levelpotential is input to the inverted input terminal NB, the transistor 110is turned on and the transistor 109 is turned off. Thus, the outputterminal OUT is connected to the power supply line V2 and the powersupply potential V2 (high power supply potential) is output from theoutput terminal OUT. When a low-level potential is input to the inputterminal N and a high-level potential is input to the inverted inputterminal NB, the transistor 110 is turned off and the transistor 109 isturned on. Thus, the output terminal OUT is connected to the powersupply line V1. Here, when the potential of the source of the transistor108 reaches a certain potential (a potential that is higher than thepower supply potential V1, which is the potential of the gate of thetransistor 108, by the threshold voltage of the transistor 108) by thelow-level potential input to the input terminal N, the transistor 108 isturned off and the source of the transistor 108 is brought into afloating state. The potential of the source of the transistor 109 keepsdecreasing, so that the potential of the gate of the transistor 109 islowered by capacitive coupling due to the capacitor 212. Accordingly,the potential of the output terminal OUT is lowered to the power supplypotential V1 (or a potential close thereto), and the power supplypotential V1 (low power supply potential) is output from the outputterminal OUT. In other words, it can be said that the amplifier circuitBUF in FIG. 5 includes a bootstrap circuit.

The amplifier circuit BUF operates in the above manner and thus can becalled a buffer circuit or a level shifter circuit.

Note that each of the logic circuits INV1 to INV4 illustrated in FIG. 4can have the same configuration as the logic circuit NV illustrated inFIG. 2B in Embodiment 1. As has been described in Embodiment 1, thelogic circuit NV in FIG. 2B can have a simpler configuration than thelogic circuit NV in FIG. 2A, but cannot output an output signal with apredetermined amplitude, which is equal to the power supply voltage(corresponding to a difference between the high power supply potentialand the low power supply potential, that is, |V1−V2|).

However, in FIG. 4, the outputs from the logic circuits INV3 and INV4are input through the switches SW3 and SW4 to the logic circuits INV1and INV2. The outputs from the logic circuits INV1 and INV2 areamplified by the amplifier circuits BUF1 and BUF2, and the amplifiedsignals are to be the output signals from the output terminal OUT andthe inverted output terminal OUTB of each stage 10. For that reason,even if the logic circuits INV1 to INV4 cannot output an output signalwith an amplitude that is (substantially) equal to the power supplyvoltage (corresponding to a difference between the high power supplypotential and the low power supply potential, that is, |V1−V2|), outputsignals with an amplitude that is (substantially) equal to the powersupply voltage are output from the output terminal OUT and the invertedoutput terminal OUTB of the stage 10. Consequently, a semiconductordevice that can output a signal with a predetermined amplitude can beprovided.

Alternatively, any or all of the logic circuits INV1 to INV4 in FIG. 4can have the same configuration as the logic circuit NV illustrated inFIG. 2A in Embodiment 1.

Each of the switches SW1 and SW2 can have the same configuration as thatillustrated in FIG. 2C in Embodiment 1, and the description is thereforenot repeated. The switches SW3 and SW4 can have the same configurationas that illustrated in FIG. 2D in Embodiment 1; therefore thedescription is not repeated.

Here, the requisite current drive capability of the switch is lower thanthat of the logic circuit and the amplifier circuit, so that the size ofthe transistor composed of the switch can be made smaller than that ofthe transistors included in the logic circuit and the amplifier circuit.That is, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors 106 and 107 can be designed to besmaller than that of any or all of the transistors 104, 105, 108, 109,and 110. Thus, the increase in integration and reduction in size of thesemiconductor device can be achieved.

Since the logic circuits INV1 to INV4 do not directly drive a loadconnected to the output terminal OUT, their requisite current drivecapability is lower than that of the amplifier circuits BUF1 and BUF2.Therefore, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors included in the logic circuitsINV1 to INV4 (the transistors 104 and 105) can be smaller than that ofthe transistors included in the amplifier circuits BUF1 and BUF2 (thetransistors 108 to 110, in particular the transistors 109 and 110).Thus, the increase in integration and reduction in size of thesemiconductor device can be achieved.

The stage 10 illustrated in FIG. 4 has a configuration in which thelogic circuit INV1, the amplifier circuit BUF1, the logic circuit INV2,and the amplifier circuit BUF2 are provided instead of the logiccircuits INV1 and INV2 in the stage 10 in FIG. 1. In other words, thestage 10 in FIG. 4 has a configuration in which each of the logiccircuits INV1 and INV2 in the stage 10 in FIG. 1 is divided into acircuit having a logic level inversion function and a circuit having anamplification function. Separation of the circuit having a logic levelinversion function and the circuit having an amplification functionenables transistors constituting the circuit having a logic levelinversion function (the logic circuits INV1 and INV2 in FIG. 4) to besmaller in size than transistors constituting the circuit having anamplification function (the amplifier circuits BUF1 and BUF2 in FIG. 4)as described above. Consequently, the current drive capability ofcircuits (e.g., the switches SW1 to SW4 in FIG. 4) which input signalsto the circuits having a logic level inversion function (the logiccircuits INV1 and INV2 in FIG. 4) can be low. Thus, the increase inintegration and reduction in size of the semiconductor device can beachieved.

Since the transistor 108 in the amplifier circuit BUF illustrated inFIG. 5 does not directly drive a load connected to the output terminalOUT, its requisite current drive capability is lower than that of thetransistors 109 and 110. For that reason, the channel width (W) (or theratio (W/L) of channel width to channel length (L)) of the transistor108 can be made smaller than that of the transistors 109 and 110.

The semiconductor device according to one embodiment of the presentinvention can include a plurality of stages 10 illustrated in FIG. 4.For example, as illustrated in FIG. 3, the semiconductor device 100 caninclude a plurality of stages 10 that are cascaded so that the inputterminal N is connected to the output terminal OUT of the previous stageand the inverted input terminal NB is connected to the inverted outputterminal OUTB of the previous stage. Here, in adjacent two stages amongthe plurality of stages 10, the logic levels of signals input to theterminals C1 can be different from each other (i.e., the logic levels ofsignals input to the terminals C2 can be different from each other). Forexample, the clock signal can be input to the terminal C1 and theinverted clock signal can be input to the terminal C2 in one stage 10,and the inverted clock signal can be input to the terminal C1 and theclock signal can be input to the terminal C2 in the stage 10 next to theone stage 10. In FIG. 3, the clock signal is represented by CLK, and theinversion signal of the clock signal is represented by CLKB. Signalsinput to the input terminal IN and the inverted input terminal NB of thefirst stage among the cascaded stages 10 can be signals with oppositelogic levels. In FIG. 3, the signal SP and the inversion signal SPB ofthe signal SP are input to the input terminal N and the inverted inputterminal INB, respectively, of the first stage.

The semiconductor device 100 illustrated in FIG. 3 has a function ofholding a signal that is input to the input terminal IN of the firststage sequentially in the stages 10 in synchronization with the clocksignal. The semiconductor device 100 can therefore be called a shiftregister. The signal SP can be called a start pulse. The semiconductordevice 100 has a function of sequentially shifting the start pulse insynchronization with the clock signal CLK and outputting the shiftedpulses as outputs SR1, SR2, SR3 . . . . The semiconductor device 100 candrive a load with the outputs SR1, SR2, SR3 . . . . Although the outputsSR1 to SR3 are set as output signals from the output terminal OUT ofeach stage 10 here, they are not limited to these and may be outputsignals from the inverted output terminal OUTB of each stage 10, forexample. Alternatively, only outputs SR2, SR4, SR6 . . . ofeven-numbered stages 10 may be used as outputs of the semiconductordevice 100 to drive a load.

The semiconductor device shown in this embodiment includes the switches(the switches SW1 to SW4), the logic circuits having an input terminalto which an input signal is input through the switch in the on state(the logic circuits INV1 to INV4), and the amplifier circuits (theamplifier circuits BUF1 and BUF2). The switch is turned on or off by aclock signal (or an inversion signal of the clock signal). The amplifiercircuit outputs a signal having the same logic level as an input signalfrom the output terminal OUT by connecting the output terminal to thehigh power supply line or the low power supply line. The output from theamplifier circuit is to be the output from the semiconductor device.Accordingly, a load driven by the semiconductor device is driven whilebeing connected to the high power supply line or the low power supplyline. The semiconductor device having such a structure does not drive aload by using a high-level potential (or a low-level potential) of theclock signal, and thus a clock signal generator does not need highcurrent drive capability. Consequently, the area of the clock signalgenerator can be reduced.

When all the transistors included in the semiconductor device have thesame conductivity type, the process of fabricating the semiconductordevice can be simplified. Thus, the yield can be increased and the costscan be reduced. In that case, the amplifier circuits (the amplifiercircuits BUF1 and BUF2) are configured to correct the output signal byusing a bootstrap circuit. In such a manner, the amplifier circuitconstituted of transistors of the same conductivity type can output anoutput signal with an amplitude voltage that is (substantially) equal tothe power supply voltage (corresponding to a difference between the highpower supply potential and the low power supply potential).

Further, the logic circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The logic circuit can be configured to invertthe logic level of an input signal input to the input terminal andoutput the inverted signal from the output terminal by controllingconnection between the output terminal and one of the high power supplyline and the low power supply line with a signal input to the invertedinput terminal, and by controlling connection between the outputterminal and the other of the high power supply line and the low powersupply line with a signal input to the input terminal. Accordingly, inthe logic circuit composed of transistors of the same conductivity type,one of the transistor provided between the high power supply line andthe output terminal and the transistor provided between the low powersupply line and the output terminal can be turned on, while the other ofthese two transistors can be turned off. As a result, the throughcurrent in a circuit provided between the high power supply line and thelow power supply line can be suppressed.

The amplifier circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The amplifier circuit can be configured tooutput, from the output terminal, a signal having the same logic levelas the input signal input to the input terminal by controllingconnection between the output terminal and one of the high power supplyline and the low power supply line with a signal input to the invertedinput terminal, and by controlling connection between the outputterminal and the other of the high power supply line and the low powersupply line with a signal input to the input terminal. Accordingly, inthe amplifier circuit composed of transistors of the same conductivitytype, one of the transistor provided between the high power supply lineand the output terminal and the transistor provided between the lowpower supply line and the output terminal can be turned on, while theother of these two transistors can be turned off. As a result, thethrough current in a circuit provided between the high power supply lineand the low power supply line can be suppressed.

The transistors included in the semiconductor device each include asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith the first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with the second gateinsulating layer placed therebetween. The other of the pair of gateelectrodes (backgate) of the transistor can be connected to the sourcethereof. Alternatively, the other of the pair of gate electrodes(backgate) can be connected to the low power supply line when thetransistor is an n-channel transistor, and can be connected to the highpower supply line when the transistor is a p-channel transistor.

Thus, the transistors can be prevented from being placed in anormally-on state, resulting in reduction in malfunction of thesemiconductor device and reduction in through current.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In Embodiment 3, a specific embodiment of a semiconductor deviceaccording to the present invention will be described with reference toFIG. 6.

The semiconductor device can include a stage 10 illustrated in FIG. 6.The stage 10 includes an input terminal N, an inverted input terminalNB, a switch SW1, a switch SW2, a switch SW3, a switch SW4, a logiccircuit INV1, a logic circuit INV2, a logic circuit INV3, a logiccircuit INV4, an amplifier circuit BUF1, an amplifier circuit BUF2, anamplifier circuit BUF3, an amplifier circuit BUF4, an output terminalOUT, and an inverted output terminal OUTB.

Each of the logic circuits INV1 to INV4 has an input terminal N, aninverted input terminal NB, and an output terminal OUT and inverts asignal input to the input terminal N and outputs the inverted signalfrom the output terminal OUT. The logic circuits INV1 to INV4 can becalled inverters. Each of the amplifier circuits BUF1 to BUF4 has aninput terminal N, an inverted input terminal NB, and an output terminalOUT and transforms (lowers) the impedance to output, from the outputterminal OUT, a signal input to the input terminal N. The outputterminal OUT of the logic circuit INV1 is connected to the inputterminal N of the amplifier circuit BUF1 and the inverted input terminalNB of the amplifier circuit BUF2. The output terminal OUT of the logiccircuit INV2 is connected to the inverted input terminal NB of theamplifier circuit BUF1 and the input terminal N of the amplifier circuitBUF2. The output terminal OUT of the amplifier circuit BUF1 is connectedto the input terminal N of the logic circuit INV3, the inverted inputterminal INB of the logic circuit INV4, and the output terminal OUT. Theoutput terminal OUT of the amplifier circuit BUF2 is connected to theinverted input terminal NB of the logic circuit INV3, the input terminalN of the logic circuit INV4, and the inverted output terminal OUTB. Theoutput terminal OUT of the logic circuit INV3 is connected to the inputterminal N of the amplifier circuit BUF3 and the inverted input terminalNB of the amplifier circuit BUF4. The output terminal OUT of the logiccircuit INV4 is connected to the inverted input terminal NB of theamplifier circuit BUF3 and the input terminal N of the amplifier circuitBUF4. The switches SW1 to SW4 each have a function of determiningelectrical continuity between a terminal A and a terminal B depending ona signal input to a terminal X. Thus, the switch SW1 has a function ofdetermining electrical continuity between the input terminal N and theinput terminal N of the logic circuit INV1 and between the inputterminal N and the inverted input terminal NB of the logic circuit INV2depending on a signal input to the terminal X. The switch SW2 has afunction of determining electrical continuity between the inverted inputterminal NB and the inverted input terminal NB of the logic circuit INV1and between the inverted input terminal NB and the input terminal IN ofthe logic circuit INV2 depending on a signal input to the terminal X.The terminals X of the switches SW1 and SW2 are connected to a terminalC1 to which one of a clock signal and an inversion signal of the clocksignal (inverted clock signal) is input. The switch SW3 has a functionof determining electrical continuity between the output terminal OUT ofthe amplifier circuit BUF3 and the input terminal N of the logic circuitINV1 and between the output terminal OUT of the amplifier circuit BUF3and the inverted input terminal NB of the logic circuit INV2 dependingon a signal input to the terminal X. The switch SW4 has a function ofdetermining electrical continuity between the output terminal OUT of theamplifier circuit BUF4 and the inverted input terminal NB of the logiccircuit INV1 and between the output terminal OUT of the amplifiercircuit BUF4 and the input terminal N of the logic circuit INV2depending on a signal input to the terminal X. The terminals X of theswitches SW3 and SW4 are connected to a terminal C2 to which the otherof the clock signal and the inverted clock signal is input.

In the stage 10 illustrated in FIG. 6, when the switches SW1 and SW2 areturned on by the clock signal (or the inverted clock signal), theswitches SW3 and SW4 are turned off by the inverted clock signal (or theclock signal), whereas when the switches SW1 and SW2 are turned off bythe clock signal (or the inverted clock signal), the switches SW3 andSW4 are turned on by the inverted clock signal (or the clock signal).Accordingly, the stage 10 has a function of holding a signal input tothe input terminal N in synchronization with the clock signal. The stage10 illustrated in FIG. 6 can therefore be called a flip-flop circuit ora latch circuit.

Specifically, each of the amplifier circuits BUF1 to BUF4 can have thesame configuration as the amplifier circuit BUF illustrated in FIG. 5 inEmbodiment 2; therefore, the description is not repeated.

Note that each of the logic circuits INV1 to INV4 illustrated in FIG. 6can have the same configuration as the logic circuit NV illustrated inFIG. 2B in Embodiment 1. As has been described in Embodiment 1, thelogic circuit NV in FIG. 2B can have a simpler configuration than thelogic circuit NV in FIG. 2A, but cannot output an output signal with apredetermined amplitude, which is equal to the power supply voltage(corresponding to a difference between the high power supply potentialand the low power supply potential, that is, |V1−V2|).

However, in FIG. 6, the outputs from the logic circuits INV1 and INV2are amplified by the amplifier circuits BUF1 and BUF2, and the amplifiedsignals are to be the output signals from the output terminal OUT andthe inverted output terminal OUTB of each stage. The outputs from thelogic circuits INV3 and INV4 are amplified by the amplifier circuitsBUF3 and BUF4. For that reason, even if the logic circuits INV1 to INV4cannot output an output signal with an amplitude that is (substantially)equal to the power supply voltage (corresponding to a difference betweenthe high power supply potential and the low power supply potential, thatis, |V1−V2|), output signals with an amplitude that is (substantially)equal to the power supply voltage are output from the output terminalOUT and the inverted output terminal OUTB of the stage 10. Consequently,a semiconductor device that can output a signal with a predeterminedamplitude can be provided.

In addition, in the configuration illustrated in FIG. 6, signals inputto the logic circuits INV1 and INV2 through the switches SW3 and SW4 aresignals with an amplitude that is (substantially) equal to the powersupply voltage (corresponding to a difference between the high powersupply potential and the low power supply potential, that is, |V1−V2|),which are amplified by the amplifier circuits BUF3 and BUF4. For thatreason, a malfunction of the logic circuits INV1 and INV2 can be reducedeven if the threshold voltage of the transistors included in these logiccircuits changes somewhat due to deterioration over time or the like.

Alternatively, any or all of the logic circuits INV1 to INV4 in FIG. 6can have the same configuration as the logic circuit NV illustrated inFIG. 2A in Embodiment 1.

Each of the switches SW1 and SW2 can have the same configuration as thatillustrated in FIG. 2C in Embodiment 1, and the description is thereforenot repeated. The switches SW3 and SW4 can have the same configurationas that illustrated in FIG. 2D in Embodiment 1; therefore thedescription is not repeated.

Here, the requisite current drive capability of the switch is lower thanthat of the logic circuit and the amplifier circuit, so that the size ofthe transistor composed of the switch can be made smaller than that ofthe transistors included in the logic circuit and the amplifier circuit.That is, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors 106 and 107 can be designed to besmaller than that of any or all of the transistors 104, 105, and 108 to110. Thus, the increase in integration and reduction in size of thesemiconductor device can be achieved.

Since the logic circuits INV1 to INV4 do not directly drive a loadconnected to the output terminal OUT, their requisite current drivecapability is lower than that of the amplifier circuits BUF1 and BUF2.Therefore, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors included in the logic circuitsINV1 to INV4 (the transistors 104 and 105) can be smaller than that ofthe transistors included in the amplifier circuits BUF1 and BUF2 (thetransistors 108 to 110, in particular the transistors 109 and 110).Thus, the increase in integration and reduction in size of thesemiconductor device can be achieved.

Since the transistor 108 in the amplifier circuit BUF illustrated inFIG. 5 does not directly drive a load connected to the output terminalOUT, its requisite current drive capability is lower than that of thetransistors 109 and 110. For that reason, the channel width (W) (or theratio (W/L) of channel width to channel length (L)) of the transistor108 can be made smaller than that of the transistors 109 and 110.

Since the amplifier circuits BUF3 and BUF4 do not directly drive a loadconnected to the output terminal OUT, their requisite current drivecapability is lower than that of the amplifier circuits BUF1 and BUF2.Therefore, the channel width (W) (or the ratio (W/L) of channel width tochannel length (L)) of the transistors included in the amplifiercircuits BUF3 and BUF4 (the transistors 108 to 110, in particular thetransistors 109 and 110) can be smaller than that of the transistorsincluded in the amplifier circuits BUF1 and BUF2 (the transistors 108 to110, in particular the transistors 109 and 110). Thus, the increase inintegration and reduction in size of the semiconductor device can beachieved.

The semiconductor device according to one embodiment of the presentinvention can include a plurality of stages 10 illustrated in FIG. 6.For example, as illustrated in FIG. 3, the semiconductor device 100 caninclude a plurality of stages 10 that are cascaded so that the inputterminal N is connected to the output terminal OUT of the previous stageand the inverted input terminal NB is connected to the inverted outputterminal OUTB of the previous stage. Here, in adjacent two stages amongthe plurality of stages 10, the logic levels of signals input to theterminals C1 can be different from each other (i.e., the logic levels ofsignals input to the terminals C2 can be different from each other). Forexample, the clock signal can be input to the terminal C1 and theinverted clock signal can be input to the terminal C2 in one stage 10,and the inverted clock signal can be input to the terminal C1 and theclock signal can be input to the terminal C2 in the stage 10 next to theone stage 10. In FIG. 3, the clock signal is represented by CLK, and theinversion signal of the clock signal is represented by CLKB. Signalsinput to the input terminal IN and the inverted input terminal NB of thefirst stage among the cascaded stages 10 can be signals with oppositelogic levels. In FIG. 3, the signal SP and the inversion signal SPB ofthe signal SP are input to the input terminal N and the inverted inputterminal INB, respectively, of the first stage.

The semiconductor device 100 illustrated in FIG. 3 has a function ofholding a signal that is input to the input terminal N of the firststage sequentially in the stages 10 in synchronization with the clocksignal. The semiconductor device 100 can therefore be called a shiftregister. The signal SP can be called a start pulse. The semiconductordevice 100 has a function of sequentially shifting the start pulse insynchronization with the clock signal CLK and outputting the shiftedpulses as outputs SR1, SR2, SR3 . . . . The semiconductor device 100 candrive a load with the outputs SR1, SR2, SR3 . . . . Although the outputsSR1 to SR3 are set as output signals from the output terminal OUT ofeach stage 10 here, they are not limited to these and may be outputsignals from the inverted output terminal OUTB of each stage 10, forexample. Alternatively, only outputs SR2, SR4, SR6 . . . ofeven-numbered stages 10 may be used as outputs of the semiconductordevice 100 to drive a load.

The semiconductor device shown in this embodiment includes the switches(the switches SW1 to SW4), the logic circuits having an input terminalto which an input signal is input through the switch in the on state(the logic circuits INV1 to INV4), and the amplifier circuits (theamplifier circuits BUF1 to BUF4). The switch is turned on or off by aclock signal (or an inversion signal of the clock signal). The amplifiercircuit outputs a signal having the same logic level as an input signalfrom the output terminal OUT by connecting the output terminal to thehigh power supply line or the low power supply line. The output from theamplifier circuit is to be the output from the semiconductor device.Accordingly, a load driven by the semiconductor device is driven whilebeing connected to the high power supply line or the low power supplyline. The semiconductor device having such a structure does not drive aload by using a high-level potential (or a low-level potential) of theclock signal, and thus a clock signal generator does not need highcurrent drive capability. Consequently, the area of the clock signalgenerator can be reduced.

When all the transistors included in the semiconductor device have thesame conductivity type, the process of fabricating the semiconductordevice can be simplified. Thus, the yield can be increased and the costscan be reduced. In that case, the amplifier circuits (the amplifiercircuits BUF1 to BUF4) are configured to correct the output signal byusing a bootstrap circuit. In such a manner, the amplifier circuitconstituted of transistors of the same conductivity type can output anoutput signal with an amplitude voltage that is (substantially) equal tothe power supply voltage (corresponding to a difference between the highpower supply potential and the low power supply potential).

Further, the logic circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The logic circuit can be configured to invertthe logic level of an input signal input to the input terminal andoutput the inverted signal from the output terminal by controllingconnection between the output terminal and one of the high power supplyline and the low power supply line with a signal input to the invertedinput terminal, and by controlling connection between the outputterminal and the other of the high power supply line and the low powersupply line with a signal input to the input terminal. Accordingly, inthe logic circuit composed of transistors of the same conductivity type,one of the transistor provided between the high power supply line andthe output terminal and the transistor provided between the low powersupply line and the output terminal can be turned on, while the other ofthese two transistors can be turned off. As a result, the throughcurrent in a circuit provided between the high power supply line and thelow power supply line can be suppressed.

The amplifier circuit has a plurality of input terminals (an inputterminal to which an input signal is input, and an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input). The amplifier circuit can be configured tooutput, from the output terminal, a signal having the same logic levelas the input signal input to the input terminal by controllingconnection between the output terminal and one of the high power supplyline and the low power supply line with a signal input to the invertedinput terminal, and by controlling connection between the outputterminal and the other of the high power supply line and the low powersupply line with a signal input to the input terminal. Accordingly, inthe amplifier circuit composed of transistors of the same conductivitytype, one of the transistor provided between the high power supply lineand the output terminal and the transistor provided between the lowpower supply line and the output terminal can be turned on, while theother of these two transistors can be turned off. As a result, thethrough current in a circuit provided between the high power supply lineand the low power supply line can be suppressed.

The transistors included in the semiconductor device each include asemiconductor layer in which a channel is formed and a pair of gateelectrodes between which the semiconductor layer is sandwiched. One ofthe pair of gate electrodes can overlap with the semiconductor layerwith the first gate insulating layer placed therebetween, and the otherthereof can overlap with the semiconductor layer with the second gateinsulating layer placed therebetween. The other of the pair of gateelectrodes (backgate) of the transistor can be connected to the sourcethereof. Alternatively, the other of the pair of gate electrodes(backgate) can be connected to the low power supply line when thetransistor is an n-channel transistor, and can be connected to the highpower supply line when the transistor is a p-channel transistor. Thus,the transistors can be prevented from being placed in a normally-onstate, resulting in reduction in malfunction of the semiconductor deviceand reduction in through current.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In Embodiment 4, an embodiment of the structure of a transistor includedin the semiconductor device shown in Embodiments 1 to 3 (e.g., thetransistors 101 to 110) will be described with reference to FIGS. 8A and8B and FIGS. 9A to 9E.

FIG. 8A is a top view of a transistor. FIG. 8B is an example of across-sectional view along A1-A2 in FIG. 8A. The transistor in FIGS. 8Aand 8B includes a layer containing an oxide semiconductor (hereinafterreferred to as “oxide semiconductor layer”) as a semiconductor layer inwhich a channel is formed. Advantages of using an oxide semiconductorlayer are that high mobility and low off-state current can be achievedby a simple low-temperature process.

As illustrated in FIG. 8B, a transistor 410 includes a gate electrode402, a gate insulating layer 404, an oxide semiconductor layer 412, adrain electrode 414 a, and a source electrode 414 b over a substrate 400having an insulating surface. Moreover, a gate insulating layer 416 incontact with the oxide semiconductor layer 412 is provided, and a gateelectrode 418 is provided over the gate insulating layer 416. Note thatone of the gate electrodes 402 and 418 corresponds to the gate inEmbodiments 1 to 3, and the other of the gate electrodes 402 and 418corresponds to the backgate in Embodiments 1 to 3. The source electrode414 b corresponds to the source in Embodiments 1 to 3, and the drainelectrode 414 a corresponds to the drain in Embodiments 1 to 3.

It is possible that one of the gate electrodes 402 and 418 serves as thegate and the other thereof serves as the backgate in one or more of aplurality of transistors included in the semiconductor device, and thatone of the gate electrodes 402 and 418 serves as the backgate and theother thereof serves as the gate in the other transistors. That is, thepositional relation between the gate and backgate of one or more of thetransistors can be opposite to that of the other transistors included inthe semiconductor device. For example, some of the transistors includedin the semiconductor device can be bottom-gate transistors in which thegate electrode 402 provided below the oxide semiconductor layer 412serves as a gate, and the other transistors included in thesemiconductor device can be top-gate transistors in which the gateelectrode 418 provided above the oxide semiconductor layer 412 serves asa gate. The backgate of the transistor (the gate electrode 402 or thegate electrode 418 depending on the structure of the transistor) can beelectrically connected to the source thereof. Alternatively, thebackgate can be electrically connected to a low power supply line whenthe transistor is an n-channel transistor, and electrically connected toa high power supply line when the transistor is a p-channel transistor.

In this embodiment, the oxide semiconductor layer 412 is used as thesemiconductor layer. Since the off-state current of the transistor 410including the oxide semiconductor layer 412 can be extremely low, theuse of the transistor 410 in a shift register or the like facilitatesretention of the potential of each node in the circuit, resulting in avery low probability of occurrence of malfunctions.

In the transistor 410, the other of the gate electrodes 402 and 418(backgate) can be connected to the source electrode 414 b. Note that thesource electrode 414 b may be connected to the low power supply linewhen the transistor 410 is an n-channel transistor. Thus, the transistor410 can be prevented from being placed in a normally-on state.Alternatively, the gate electrode 402 and the gate electrode 418 may beconnected to each other to be used as the gate of the transistor 410.

The transistor 410 illustrated in FIGS. 8A and 8B has a structure wherethe drain electrode 414 a and the source electrode 414 b partly overlapwith the gate electrode 402, or alternatively may have a structure wherethe drain electrode 414 a and the source electrode 414 b do not overlapwith the gate electrode 402.

There is no particular limitation on the crystallinity of the oxidesemiconductor layer 412. For example, the oxide semiconductor layer 412can be non-single-crystal and may be amorphous or polycrystalline.Alternatively, the oxide semiconductor layer 412 may have an amorphousstructure including a crystalline portion.

An amorphous oxide semiconductor layer can have a flat surface withrelative ease; therefore, by using an amorphous oxide semiconductorlayer to fabricate a transistor, interface scattering can be reduced,and relatively high mobility can be obtained with relative ease.

In a crystalline oxide semiconductor layer, defects in the bulk can befurther reduced. When the flatness of the surface of the crystallineoxide semiconductor layer 412 is increased, the crystalline oxidesemiconductor layer 412 can have higher mobility than an amorphous oxidesemiconductor layer. In order to increase the surface flatness, theoxide semiconductor is preferably formed on a flat surface.Specifically, the oxide semiconductor is preferably formed on a surfacewith an average surface roughness (Ra) of 1 nm or less, preferably 0.3nm or less, further preferably 0.1 nm or less.

Note that Ra is obtained by three-dimension expansion of arithmetic meansurface roughness that is defined by JIS B 0601:2001 (ISO 4287:1997) soas to be applied to a curved surface. In addition, Ra is an averagevalue of the absolute values of deviations from a reference surface to aspecific surface and is defined by the following formula.

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \mspace{619mu}} & \; \\{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {x}\ {y}}}}}} & (1)\end{matrix}$

Here, the specific surface is a surface that is a target of roughnessmeasurement, and is a quadrilateral region specified by four pointsrepresented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂, f(x₁, y₂)),(x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). Moreover, S₀ representsthe area of a rectangle which is obtained by projecting the specificsurface on the xy plane, and Z₀ represents the height of the referencesurface (the average height of the specific surface). Further, Ra can bemeasured with an atomic force microscope (AFM).

The oxide semiconductor layer 412 can be a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts are includedin an amorphous phase. Note that in most cases, the crystal part fitsinside a cube whose one side is less than 100 nm. From an observationimage obtained with a transmission electron microscope (TEM), a boundarybetween an amorphous part and a crystal part in the CAAC-OS film is notclear. Further, a grain boundary in the CAAC-OS film is not found withthe TEM. Thus, a reduction in electron mobility due to the grainboundary is suppressed in the CAAC-OS film.

In each of the crystal parts included in the CAAC-OS film, the c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a top surface of theCAAC-OS film, triangular or hexagonal atomic arrangement is formed whenseen from the direction perpendicular to the a-b plane, and metal atomsare arranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part. In this specification, the term “perpendicular”includes a range from 85° to 95°, and the term “parallel” includes arange from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, when crystal growth occurs from a top surface sideof the oxide semiconductor film in the process of forming the CAAC-OSfilm, the proportion of crystal parts in the vicinity of the top surfaceof the oxide semiconductor film is sometimes higher than that in thevicinity of the surface where the oxide semiconductor film is formed.Further, when an impurity is added to the CAAC-OS film, the crystal partin a region to which the impurity is added becomes amorphous in somecases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of the surfacewhere the CAAC-OS film is formed or a normal vector of the top surfaceof the CAAC-OS film, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS film (thecross-sectional shape of the surface where the CAAC-OS film is formed orthe cross-sectional shape of the top surface of the CAAC-OS film). Notethat the direction of the c-axis of the crystal part is the directionparallel to a normal vector of the surface where the CAAC-OS film isformed or a normal vector of the top surface of the CAAC-OS film. Thecrystal part is formed by deposition or by performing treatment forcrystallization such as heat treatment after deposition.

In a transistor including the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight can be reduced; thus, the transistor has high reliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

There are three methods for obtaining a CAAC-OS film. The first is amethod in which an oxide semiconductor layer is deposited attemperatures ranging from 200° C. to 500° C. so that the c-axes ofcrystals are substantially perpendicular to the top surface. The secondis a method in which a thin oxide semiconductor layer is deposited andthen subjected to heat treatment at temperatures ranging from 200° C. to700° C. so that the c-axes of crystals are substantially perpendicularto the top surface. The third is a method in which a thin oxidesemiconductor layer is deposited as a first layer and then subjected toheat treatment at temperatures ranging from 200° C. to 700° C. and anoxide semiconductor layer is deposited thereover as a second layer sothat the c-axes of crystals are substantially perpendicular to the topsurface.

The oxide semiconductor layer 412 has a thickness of 1 nm to 30 nm(preferably 5 nm to 10 nm) and can be formed by sputtering, molecularbeam epitaxy (MBE), CVD, pulsed laser deposition, atomic layerdeposition (ALD), or the like as appropriate. Alternatively, the oxidesemiconductor layer 412 may be formed using a sputtering apparatus thatperforms deposition with surfaces of a plurality of substrates setsubstantially perpendicular to a surface of a sputtering target.

The oxide semiconductor layer 412 used as the semiconductor layer inthis embodiment is preferably purified by removal of hydrogen, which isan n-type impurity, from the oxide semiconductor so that impuritieswhich are not main components of the oxide semiconductor are containedas little as possible.

Note that the purified oxide semiconductor layer has very few carriers,and its carrier density is less than 1×10¹⁴/cm³, preferably less than1×10¹²/cm³, further preferably less than 1×10¹¹/cm³. Such few carriersenable a current in an off state (off-state current) to be sufficientlylow.

Specifically, in the transistor including the above-described oxidesemiconductor layer, the off-state current density obtained by dividingthe off-state current by the channel width of the transistor at roomtemperature (25° C.) can be 100 zA/μm (1×10⁻¹⁹ A/μm) or lower, or even10 zA/μm (1×10⁻²° A/μm) or lower under conditions that the channellength L of the transistor is 10 μm and the source-drain voltage is 3 V.

The transistor 410 including the purified oxide semiconductor layer haslittle temperature dependence of on-state current, and its off-statecurrent remains low even at high temperatures.

Next, a process of fabricating the transistor 410 illustrated in FIGS.8A and 8B will be described with reference to FIGS. 9A to 9E.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then the gate electrode 402 is formed in aphotolithography process. Note that a resist mask used in thephotolithography process may be formed by an inkjet method. Formation ofthe resist mask by an inkjet method needs no photomask; thus,fabrication costs can be reduced.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. Alternatively, thesubstrate 400 may be a single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon, silicon carbide, orthe like; a compound semiconductor substrate of silicon germanium or thelike; an SOI substrate; or any of these substrates over which asemiconductor element is provided.

A semiconductor device may be fabricated using a flexible substrate asthe substrate 400. In order to fabricate a flexible semiconductordevice, the transistor 410 including the oxide semiconductor layer 412may be directly formed over a flexible substrate. Alternatively, thetransistor 410 including the oxide semiconductor layer 412 may be formedover a substrate, and then the transistor 410 may be separated andtransferred to a flexible substrate. In order to separate the transistorfrom the substrate and transfer it to the flexible substrate, aseparation layer may be provided between the substrate and thetransistor 410 including the oxide semiconductor layer 412.

The gate electrode 402 can be formed using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material containing any of thesematerials as its main component. Alternatively, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the gate electrode 402. The gate electrode 402 mayhave a single-layer structure or a layered structure.

The gate electrode 402 can also be formed using a conductive materialsuch as indium tin oxide, indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, indium zinc oxide, orindium tin oxide to which silicon oxide is added. Alternatively, thegate electrode 402 can have a layered structure of the above conductivematerial and the above metal material.

As one layer of the gate electrode 402 which is in contact with the gateinsulating layer 404 formed later, a film of metal oxide containingnitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, anIn—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen,an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, anIn—O film containing nitrogen, or a metal nitride (e.g., InN or SnN)film can be used. These films have a work function of 5 eV or higher,preferably 5.5 eV or higher; thus, when any of these films is used asthe gate electrode, the threshold voltage of the n-channel transistorcan be positive. Accordingly, a normally-off switching element can beprovided.

Next, the gate insulating layer 404 is formed over the gate electrode402.

The gate insulating layer 404 has a thickness of 1 nm to 20 nm and canbe formed by sputtering, MBE, CVD, pulsed laser deposition, ALD, or thelike as appropriate. Alternatively, the gate insulating layer 404 may beformed using a sputtering apparatus that performs deposition withsurfaces of a plurality of substrates set substantially perpendicular toa surface of a sputtering target.

The gate insulating layer 404 can be formed using a silicon oxide film,a gallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film. A portion of the gate insulating layer 404 which isin contact with an oxide semiconductor layer 406 formed later preferablycontains oxygen. In particular, the gate insulating layer 404 preferablycontains a large amount of oxygen which exceeds at least thestoichiometric ratio in the layer (the bulk). For example, when asilicon oxide film is used as the gate insulating layer 404, thecomposition formula is SiO_(2+α) (α>0). In this embodiment, a siliconoxide film of SiO_(2+α) (α>0) is used as the gate insulating layer 404.By using the silicon oxide film as the gate insulating layer 404, oxygencan be supplied to the oxide semiconductor layer 406 formed later.Further, the gate insulating layer 404 is preferably formed inconsideration of the size of a transistor to be fabricated and the stepcoverage with the gate insulating layer 404.

Alternatively, the gate insulating layer 404 can be formed using ahigh-k material such as hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSiO_(x)N_(y) (x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0,y>0)), or lanthanum oxide, whereby gate leakage current can be reduced.The gate insulating layer 404 may have a single-layer structure or alayered structure.

Then, the oxide semiconductor layer 406 is formed over the gateinsulating layer 404 (see FIG. 9A).

In order to prevent the oxide semiconductor layer 406 from containinghydrogen or water as much as possible in the step of forming the oxidesemiconductor layer 406, it is preferable to heat the substrate providedwith the gate insulating layer 404 in a preheating chamber in asputtering apparatus as a pretreatment for formation of the oxidesemiconductor layer 406 so that impurities such as hydrogen and moistureadsorbed on the substrate 400 and the gate insulating layer 404 areeliminated and removed. Note that a cryopump is preferable as anexhaustion unit provided in the preheating chamber.

Planarization treatment may be performed on a region of the gateinsulating layer 404 which is in contact with the oxide semiconductorlayer 406. Although not particularly limited, the planarizationtreatment can be dry etching, plasma treatment, or polishing (e.g.,chemical mechanical polishing (CMP)).

As plasma treatment, reverse sputtering in which an argon gas isintroduced and plasma is generated can be performed, for example. Thereverse sputtering is a method in which voltage is applied to thesubstrate side with use of an RF power source in an argon atmosphere andplasma is generated in the vicinity of the substrate so that a substratesurface is modified. Note that instead of an argon atmosphere, anitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or thelike may be used. The reverse sputtering can remove particle substances(also referred to as particles or dust) attached to the top surface ofthe gate insulating layer 404.

As the planarization treatment, dry etching, plasma treatment, orpolishing may be performed plural times or these treatments may beperformed in combination. In the case where the treatments are performedin combination, there is no particular limitation on the order of stepsand the order can be set as appropriate depending on the roughness ofthe top surface of the gate insulating layer 404.

Note that the oxide semiconductor layer 406 is preferably depositedunder conditions such that much oxygen is contained (e.g., deposited bysputtering in a 100% oxygen atmosphere) so as to be a film containingmuch oxygen (preferably having a region where the oxygen content ishigher than that in the stoichiometric composition ratio of the oxidesemiconductor in a crystalline state).

An oxide semiconductor used for the oxide semiconductor layer 406preferably contains at least indium (In) or zinc (Zn). In particular,the oxide semiconductor preferably contains In and Zn. In addition, as astabilizer for reducing variations in electric characteristics oftransistors using the oxide semiconductor, the oxide semiconductorpreferably contains gallium (Ga), tin (Sn), hafnium (Hf), and/oraluminum (Al).

As another stabilizer, the oxide semiconductor may contain one or pluralkinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium(Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and/or lutetium (Lu).

As the oxide semiconductor, any of the following oxides can be used, forexample: indium oxide, tin oxide, zinc oxide, two-component metal oxidessuch as In—Zn-based oxide, Sn—Zn-based oxide, Al—Zn-based oxide,Zn—Mg-based oxide, Sn—Mg-based oxide, In—Mg-based oxide, and In—Ga-basedoxide; three-component metal oxides such as In—Ga—Zn-based oxide (alsoreferred to as IGZO), In—Al—Zn-based oxide, In—Sn—Zn-based oxide,Sn—Ga—Zn-based oxide, Al—Ga—Zn-based oxide, Sn—Al—Zn-based oxide,In—Hf—Zn-based oxide, In—La—Zn-based oxide, In—Ce—Zn-based oxide,In—Pr—Zn-based oxide, In—Nd—Zn-based oxide, In—Sm—Zn-based oxide,In—Eu—Zn-based oxide, In—Gd—Zn-based oxide, In—Tb—Zn-based oxide,In—Dy—Zn-based oxide, In—Ho—Zn-based oxide, In—Er—Zn-based oxide,In—Tm—Zn-based oxide, In—Yb—Zn-based oxide, and In—Lu—Zn-based oxide;and four-component oxides such as In—Sn—Ga—Zn-based oxide,In—Hf—Ga—Zn-based oxide, In—Al—Ga—Zn-based oxide, In—Sn—Al—Zn-basedoxide, In—Sn—Hf—Zn-based oxide, and In—Hf—Al—Zn-based oxide.

Here, for example, an In—Ga—Zn-based oxide refers to an oxide containingIn, Ga, and Zn, and there is no limitation on the composition ratio ofIn, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metal elementother than In, Ga, and Zn.

In this embodiment, as a target used for forming the oxide semiconductorlayer 406 by sputtering, an oxide target having a composition ratio ofIn:Ga:Zn=3:1:2 (atomic ratio) is used to form an In—Ga—Zn-based oxidefilm (IGZO film).

The relative density of the target ranges from 90% to 100%, preferablyfrom 95% to 99.9%. With the use of a target with high relative density,a dense oxide semiconductor layer 406 can be formed.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or hydride are removed as asputtering gas used for forming the oxide semiconductor layer 406.

The substrate is held in a deposition chamber kept under reducedpressure. Then, a sputtering gas from which hydrogen and moisture havebeen removed is introduced into the deposition chamber while moistureremaining therein is removed, and the oxide semiconductor layer 406 isformed over the substrate 400 with the use of the above target. In orderto remove moisture remaining in the deposition chamber, an entrapmentvacuum pump such as a cryopump, an ion pump, or a titanium sublimationpump is preferably used. As an exhaustion unit, a turbo molecular pumpprovided with a cold trap may be used. In the deposition chamber whichis evacuated with a cryopump, a hydrogen atom, a compound containing ahydrogen atom such as water (H₂O), and the like (preferably, also acompound containing a carbon atom) are removed, whereby the impurityconcentration in the oxide semiconductor layer 406 formed in thedeposition chamber can be reduced.

The gate insulating layer 404 and the oxide semiconductor layer 406 arepreferably formed in succession without being exposed to the air. Whenthe gate insulating layer 404 and the oxide semiconductor layer 406 areformed in succession without being exposed to the air, impurities suchas hydrogen and moisture can be prevented from being adsorbed onto a topsurface of the gate insulating layer 404.

Next, heat treatment is performed to remove excess hydrogen (includingwater and a hydroxyl group) in the oxide semiconductor layer 406 (todehydrate or dehydrogenate the oxide semiconductor layer 406). The heattreatment performed on the oxide semiconductor layer 406 can form anoxide semiconductor layer 408 from which excess hydrogen is removed (seeFIG. 9B). The temperature of the heat treatment is higher than or equalto 300° C. and lower than or equal to 700° C., or lower than the strainpoint of the substrate. The heat treatment can be performed underreduced pressure, a nitrogen atmosphere, or the like. For example, thesubstrate is put in an electric furnace which is a kind of heattreatment apparatus, and the oxide semiconductor layer 406 is subjectedto the heat treatment at 450° C. for one hour in a nitrogen atmosphere.

Further, a heat treatment apparatus is not limited to an electricfurnace, and a device for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating elementmay alternatively be used. For example, a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As thehigh-temperature gas, an inert gas which does not react with an objectby heat treatment, such as nitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperatures of 650°C. to 700° C., heated for several minutes, and taken out of the inertgas.

Note that in the heat treatment, it is preferable that moisture,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. The purity of nitrogen or the rare gas suchas helium, neon, or argon which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or higher, further preferably 7N(99.99999%) or higher (i.e., the impurity concentration is preferably 1ppm or lower, further preferably 0.1 ppm or lower).

After the oxide semiconductor layer 406 is heated by the heat treatment,a high-purity oxygen gas, a high-purity dinitrogen monoxide gas, orultra dry air (with a moisture amount of 20 ppm (−55° C. by conversioninto a dew point) or less, preferably 1 ppm or less, further preferably10 ppb or less in the measurement with a dew point meter of a cavityring down laser spectroscopy (CRDS) system) may be introduced into thesame furnace. It is preferable that water, hydrogen, or the like be notcontained in the oxygen gas or the dinitrogen monoxide gas. The purityof the oxygen gas or the dinitrogen monoxide gas which is introducedinto the heat treatment apparatus is preferably 6N or higher, furtherpreferably 7N or higher (i.e., the impurity concentration of the oxygengas or the dinitrogen monoxide gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower). The oxygen gas or the dinitrogen monoxidegas acts to supply oxygen which is a main component of the oxidesemiconductor and is reduced by the step of removing impurities for thedehydration or dehydrogenation.

The heat treatment for dehydration or dehydrogenation may be performedplural times, and may also serve as another heat treatment.

It is preferable to perform the heat treatment for dehydration ordehydrogenation while the gate insulating layer 404 is covered with thefilm-shaped oxide semiconductor layer 406, which is not processed intoan island shape to be the oxide semiconductor layer 412, because oxygencontained in the gate insulating layer 404 can be prevented from beingreleased to the outside by the heat treatment.

Through the dehydration or dehydrogenation treatment, oxygen which is amain constituent of the oxide semiconductor might be eliminated and thusreduced in amount. An oxygen vacancy exists in a portion from whichoxygen is eliminated in the oxide semiconductor layer 408, and causes adonor level leading to a change in electrical characteristics of atransistor.

Accordingly, oxygen is preferably supplied to the oxide semiconductorlayer 408 after being subjected to the dehydration or dehydrogenationtreatment. By supply of oxygen to the oxide semiconductor layer 408,oxygen vacancies in the layer can be filled.

For example, when an oxide insulating layer that contains much(excessive) oxygen and serves as an oxygen supply source is used as thegate insulating layer 404 and provided in contact with the oxidesemiconductor layer 408, oxygen can be supplied from the oxideinsulating layer to the oxide semiconductor layer 408. In the abovestructure, oxygen may be supplied to the oxide semiconductor layer 408by performing heat treatment while the oxide insulating layer is incontact with at least part of the oxide semiconductor layer 408subjected to the heat treatment for dehydration or dehydrogenation.

The gate insulating layer 404, which contains much (excessive) oxygenand serves as an oxygen supply source, is provided in contact with theoxide semiconductor layer 408, whereby oxygen can be supplied from thegate insulating layer 404 to the oxide semiconductor layer 408 and thusoxygen vacancies in the oxide semiconductor layer 408 can be filled.

Next, the oxide semiconductor layer 408 is processed in aphotolithography process, thereby forming the island-shaped oxidesemiconductor layer 412 (see FIG. 9C). Note that a resist mask used inthe photolithography process may be formed by an inkjet method.Formation of the resist mask by an inkjet method needs no photomask;thus, fabrication costs can be reduced.

Etching of the oxide semiconductor layer 408 may be wet etching, dryetching, or both wet etching and dry etching. As an etchant used for wetetching of the oxide semiconductor layer 408, a mixed solution ofphosphoric acid, acetic acid, and nitric acid can be used, for example.Alternatively, ITO07N (produced by Kanto Chemical Co., Inc.) may beused.

Then, oxygen (including at least any one of oxygen radicals, oxygenatoms, and oxygen ions) may be introduced into the oxide semiconductorlayer 412 in order to supply oxygen to the layer.

Supply of oxygen to the oxide semiconductor layer 412 by introduction ofoxygen thereto can purify the oxide semiconductor layer 412. Thetransistor including the purified oxide semiconductor layer 412 iselectrically stable because a change in electric characteristics issmall.

Oxygen can be introduced by ion implantation, ion doping, plasmaimmersion ion implantation, plasma treatment, or the like.

In the step of introducing oxygen into the oxide semiconductor layer412, oxygen may be directly introduced into the oxide semiconductorlayer 412 or introduced into the oxide semiconductor layer 412 throughthe gate insulating layer 416 formed later. Ion implantation, iondoping, plasma immersion ion implantation, or the like can be used tointroduce oxygen through the gate insulating layer 416, whereas plasmatreatment or the like can also be used to introduce oxygen directly tothe exposed oxide semiconductor layer 412.

Oxygen can be introduced into the oxide semiconductor layer anytimeafter dehydration or dehydrogenation treatment is performed thereon.Further, oxygen may be introduced plural times into the dehydrated ordehydrogenated oxide semiconductor layer.

Next, a conductive film that is to be the drain electrode 414 a and thesource electrode 414 b (as well as a wiring) is formed over the oxidesemiconductor layer 412, and after that, a photolithography process isperformed to form the drain electrode 414 a and the source electrode 414b (see FIG. 9D).

The conductive film is formed using a material that can withstand heattreatment in a later step. The conductive film used for the drainelectrode 414 a and the source electrode 414 b can be, for example, ametal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W or a metal nitride film containing any of the above elements asits component (e.g., a titanium nitride film, a molybdenum nitride film,or a tungsten nitride film). Alternatively, the conductive film may havea structure in which a refractory metal film of Ti, Mo, W, or the likeor a metal nitride film thereof (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) is stacked overand/or below a metal film of Al, Cu, or the like. Further alternatively,the conductive film used for the drain electrode 414 a and the sourceelectrode 414 b may be formed using conductive metal oxide. Examples ofthe conductive metal oxide are indium oxide (In₂O₃), tin oxide (SnO₂),zinc oxide (ZnO), indium oxide-tin oxide (In₂O₃—SnO₂: ITO), indiumoxide-zinc oxide (In₂O₃—ZnO), and any of these metal oxide materialscontaining silicon or silicon oxide.

Next, the gate insulating layer 416 is formed so as to cover the oxidesemiconductor layer 412, the drain electrode 414 a, and the sourceelectrode 414 b. A material and a formation method of the gateinsulating layer 416 can be similar to those of the gate insulatinglayer 404; therefore, the detailed description is not repeated.

Then, a conductive film that is to be the gate electrode 418 is formedover the gate insulating layer 416, and after that, a photolithographyprocess is performed to form the gate electrode 418 (see FIG. 9E). Amaterial and a formation method of the gate electrode 418 can be similarto those of the gate electrode 402; therefore, the detailed descriptionis not repeated.

Through the above steps, the transistor 410 is formed (see FIG. 9E).

A protective insulating layer may be formed over the gate insulatinglayer 416 and the gate electrode 418. The protective insulating layerprevents entry of hydrogen, water, and the like from the outside. As theprotective insulating layer, a silicon nitride film or an aluminumnitride film can be used, for example. Although there is no particularlimitation on the method of forming the protective insulating layer, RFsputtering is suitable because of its high productivity.

In addition, a planarization insulating film for reducing surfaceroughness due to the transistor may be formed over the protectiveinsulating layer. For the planarization insulating film, an organicmaterial such as polyimide, acrylic, or benzocyclobutene can be used.Other than such organic materials, it is also possible to use alow-dielectric constant material (low-k material) or the like. Theplanarization insulating film may be formed from a stack of a pluralityof insulating films formed using these materials.

After the protective insulating layer or the planarization insulatingfilm is formed, heat treatment may be performed at 100° C. to 200° C.for 1 hour to 30 hours in the air.

Since the transistor, which is fabricated as described above inaccordance with this embodiment and whose channel is formed in thepurified oxide semiconductor layer, has extremely low off-state current,the use of the transistor facilitates retention of the potential of anode. Consequently, when the transistor is used in a shift register orthe like, the probability of occurrence of malfunctions can be extremelylow.

This embodiment can be freely combined with any of the otherembodiments.

Example 1

A semiconductor device according to one embodiment of the presentinvention can be used in a variety of electronic devices. Examples ofthe electronic devices are personal computers (e.g., notebook computersand desktop computers), image reproducing devices provided withrecording media (typically, devices that reproduce the content ofrecording media such as digital versatile discs (DVDs) and have displaysfor displaying the reproduced images), mobile phones, portable gameconsoles, personal digital assistants, e-book readers, cameras such asvideo cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio systems and digital audio players), copiers, facsimiles,printers, multifunction printers, automated teller machines (ATMs), andvending machines.

An example of the electronic device will be described with reference toFIGS. 10A to 10C.

FIGS. 10A and 10B illustrate a tablet-type device (hereinafter referredto as “tablet”) that can be folded in two. FIG. 10A illustrates thetablet which is open (unfolded). The tablet includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power switch 9035, a switch 9036 forswitching to power-saving mode, a fastener 9033, and an operation switch9038.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9037 that aredisplayed. Note that FIG. 10A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can displaykeyboard buttons and serve as a touch panel while the display portion9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The switch 9034 for switching display modes can switch displayorientation (e.g., between landscape mode and portrait mode) and selecta display mode (switch between monochrome display and color display),for example. With the switch 9036 for switching to power-saving mode,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet is in use, which isdetected with an optical sensor incorporated in the tablet. The tabletmay include another detection device such as a sensor for detectingorientation (e.g., a gyroscope or an acceleration sensor) in addition tothe optical sensor.

Although FIG. 10A shows the example where the display area of thedisplay portion 9631 a is the same as that of the display portion 9631b, there is no particular limitation on the display portions 9631 a and9631 b. They may differ in size and/or image quality. For example, oneof them may be a display panel that can display higher-definition imagesthan the other.

FIG. 10B illustrates the tablet which is closed. The tablet includes thehousing 9630, a solar battery 9633, a charge/discharge control circuit9634, a battery 9635, and a DC to DC converter 9636. As an example, FIG.10B illustrates the charge/discharge control circuit 9634 including thebattery 9635 and the DC to DC converter 9636.

Since the tablet can be folded in two, the housing 9630 can be closedwhen the tablet is not in use. Thus, the display portions 9631 a and9631 b can be protected, thereby providing a tablet with high enduranceand high reliability for long-term use.

The tablet illustrated in FIGS. 10A and 10B can also have a function ofdisplaying various kinds of data (e.g., a still image, a moving image,and a text image), a function of displaying a calendar, a date, thetime, or the like on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

Power can be supplied to the display portion 9631 (the display portion9631 a and/or the display portion 9631 b) and a touch panel, an imagesignal processor, and the like in the display portion 9631 by the solarbattery 9633 attached on a surface of the tablet. The solar battery 9633is preferably provided on at least one surface of the housing 9630(e.g., all or part of the rear surface assuming that a surface where thedisplay portions 9631 a and 9631 b are provided is the front surface),in which case the battery 9635 can be efficiently charged. The use of alithium ion battery as the battery 9635 brings an advantage such asreduction in size.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 10B will be described with reference to a blockdiagram in FIG. 10C. FIG. 10C illustrates the solar battery 9633, thebattery 9635, the DC to DC converter 9636, a converter 9637, switchesSW1 to SW3, and the display portion 9631. The battery 9635, the DC to DCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge/discharge control circuit 9634 illustrated inFIG. 10B.

An example of the operation performed when power is generated by thesolar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by the DCto DC converter 9636 so as to be a voltage for charging the battery9635. Then, when power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

This example can be combined with any of the above embodiments.

This application is based on Japanese Patent Applications serial No.2011-182274 filed with Japan Patent Office on Aug. 24, 2011, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a switch comprising a firsttransistor; and a logic circuit comprising: an input terminal to whichan input signal is input through the switch; an inverted input terminalto which a signal with a logic level opposite to that of the inputsignal is input; and an output terminal from which an output signal isoutput, wherein the logic circuit comprises a bootstrap circuitcomprising at least one second transistor, wherein the bootstrap circuitis electrically connected to the output terminal, wherein the firsttransistor and the second transistor have the same conductivity type,and wherein each of the first transistor and the second transistorcomprises an oxide semiconductor layer comprising a channel formationregion and a pair of gate electrodes with the oxide semiconductor layerprovided therebetween.
 2. The semiconductor device according to claim 1,wherein the logic circuit is supplied with a high power supply potentialfrom a high power supply line, wherein the logic circuit is suppliedwith a low power supply potential from a low power supply line, andwherein one of the high power supply potential and the low power supplypotential is output from the output terminal by correcting a potentialof the output terminal with the bootstrap circuit.
 3. The semiconductordevice according to claim 1, wherein the oxide semiconductor layerfurther comprises a source region, and wherein one of the pair of gateelectrodes is electrically connected to the source region.
 4. Anelectronic device comprising the semiconductor device according toclaim
 1. 5. A semiconductor device comprising: a switch comprising afirst transistor; and a logic circuit comprising: an input terminal towhich an input signal is input through the switch; an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input; and an output terminal from which an outputsignal is output, wherein the logic circuit comprises a bootstrapcircuit comprising at least one second transistor, wherein the bootstrapcircuit is electrically connected to the output terminal, wherein thefirst transistor and the second transistor have the same conductivitytype, wherein each of the first transistor and the second transistorcomprises an oxide semiconductor layer comprising a channel formationregion and a pair of gate electrodes with the oxide semiconductor layerprovided therebetween, and wherein a width of one of the pair of gateelectrodes of the first transistor is smaller than a width of one of thepair of gate electrodes of the second transistor.
 6. The semiconductordevice according to claim 5, wherein the first transistor and the secondtransistor are n-channel transistors, wherein the logic circuit issupplied with a high power supply potential from a high power supplyline, and wherein the high power supply potential is output from theoutput terminal by raising a potential of the output terminal with thebootstrap circuit.
 7. The semiconductor device according to claim 5,wherein the first transistor and the second transistor are p-channeltransistors, wherein the logic circuit is supplied with a low powersupply potential from a low power supply line, and wherein the low powersupply potential is output from the output terminal by lowering apotential of the output terminal with the bootstrap circuit.
 8. Thesemiconductor device according to claim 5, wherein the first transistorand the second transistor are n-channel transistors, wherein anelectrical connection between a high power supply line and the outputterminal is controlled by the signal with the logic level opposite tothat of the input signal, and wherein an electrical connection between alow power supply line and the output terminal is controlled by the inputsignal.
 9. The semiconductor device according to claim 5, wherein thefirst transistor and the second transistor are p-channel transistors,wherein an electrical connection between a high power supply line andthe output terminal is controlled by the input signal, and wherein anelectrical connection between a low power supply line and the outputterminal is controlled by the signal with the logic level opposite tothat of the input signal.
 10. The semiconductor device according toclaim 5, wherein the oxide semiconductor layer further comprises asource region, and wherein one of the pair of gate electrodes iselectrically connected to the source region.
 11. An electronic devicecomprising the semiconductor device according to claim
 5. 12. Asemiconductor device comprising: a switch comprising a first transistor;and a first logic circuit and a second logic circuit, each of the firstlogic circuit and the second logic circuit comprising: an input terminalto which an input signal is input through the switch; an inverted inputterminal to which a signal with a logic level opposite to that of theinput signal is input; and an output terminal from which an outputsignal is output, wherein the first logic circuit comprises a bootstrapcircuit comprising at least one second transistor, wherein the bootstrapcircuit is electrically connected to the output terminal, wherein thesecond logic circuit comprises an inverter circuit comprising at leastone third transistor, wherein the first transistor, the secondtransistor and the third transistor have the same conductivity type, andwherein each of the first transistor, the second transistor and thethird transistor comprises an oxide semiconductor layer comprising achannel formation region and a pair of gate electrodes with the oxidesemiconductor layer provided therebetween.
 13. The semiconductor deviceaccording to claim 12, wherein the first transistor, the secondtransistor and the third transistor are n-channel transistors, whereinthe first logic circuit is supplied with a high power supply potentialfrom a high power supply line, and wherein the high power supplypotential is output from the output terminal of the first logic circuitby raising a potential of the output terminal with the bootstrapcircuit.
 14. The semiconductor device according to claim 12, wherein thefirst transistor, the second transistor and the third transistor arep-channel transistors, wherein the first logic circuit is supplied witha low power supply potential from a low power supply line, and whereinthe low power supply potential is output from the output terminal of thefirst logic circuit by lowering a potential of the output terminal withthe bootstrap circuit.
 15. The semiconductor device according to claim12, wherein the first transistor, the second transistor and the thirdtransistor are n-channel transistors, wherein an electrical connectionbetween a high power supply line and the output terminal of the firstlogic circuit is controlled by the signal with the logic level oppositeto that of the input signal, and wherein an electrical connectionbetween a low power supply line and the output terminal of the firstlogic circuit is controlled by the input signal.
 16. The semiconductordevice according to claim 12, wherein the first transistor, the secondtransistor and the third transistor are p-channel transistors, whereinan electrical connection between a high power supply line and the outputterminal of the first logic circuit is controlled by the input signal,and wherein an electrical connection between a low power supply line andthe output terminal of the first logic circuit is controlled by thesignal with the logic level opposite to that of the input signal. 17.The semiconductor device according to claim 12, wherein the oxidesemiconductor layer further comprises a source region, and wherein oneof the pair of gate electrodes is electrically connected to the sourceregion.
 18. An electronic device comprising the semiconductor deviceaccording to claim 12.